Measurements Of Velocity Fluctuations Research Articles

1305 ガストフロントに伴う乱流変動(OS13-2 大気海洋の流体現象と環境流体,オーガナイズドセッション)

We tried to generate a gust front in a multi-fan type wind tunnel in order to clarify the detailed structure of a gust front. Velocity fluctuations after the passage of the gust front were investigated. The conditioning measurements of velocity fluctuations were also made with two X-wire probes. Fully developed turbulent flow was observed in the upper layer of the gust. A periodic pattern like K-H instability wave was embedded in the turbulent fluctuation near Z=200mm. On the other hand, only turbulent motion was observed near ground. Though the strong gust was found to correspond to sweep motion, it does not exist only in the lower shear layer but correlate with large scale turbulent motion across over the whole layer of the gust.

Investigation of geodesic acoustic modes and related zonal flows at TEXTOR

This paper attempts to give an overview on the study of the geodesic acoustic mode (GAM) and the related velocity oscillation of the zonal flow (ZF) at TEXTOR. The reported results are obtained from poloidal cross correlation reflectometry. The advantages of the diagnostic combined with a multi-antenna set-up, as it is installed at TEXTOR, are shortly reviewed and compared with other diagnostics used in experiments on GAMs.Furthermore, the GAM properties in ohmic discharges are reported as frequency scaling with local temperature, poloidal structure and radial distribution. GAM induced density fluctuations and related velocity oscillations are measured for a large radial range. In the context of the latter the collisionality on the GAMs at the plasma edge is investigated. The measurement of the GAM related velocity fluctuations shows that the ZF occurs in two rings at least, separated by a rational surface. The velocity oscillation allows us to estimate the effect of GAMs on the shearing rate. The influence on the shearing rate is quantitatively investigated. The GAM induced shearing rate is not able to overcome the decorrelation rate of the ambient turbulence (AT).Also the relation between AT and GAMs is analyzed. Therefore the AT is characterized and the turbulence driving the GAMs is discussed. A relation between the density scale length and the pronounced occurrence of GAMs is obtained. This is related to a narrow region of k⊥ρs values, which is characteristic for drift ballooning modes. The spectrum of the AT is analyzed in both ZF rings. A significant change in the shape of the low frequency turbulence is obtained. This may be connected with a transition from drift ballooning modes to ITG modes. The latter seems to drive the GAMs less effectively.

Influence of spacing between buildings on wind characteristics above rural and suburban areas

A wind tunnel study has been carried out to determine the influence of spacing between buildings on wind characteristics above rural and suburban type of terrain. Experiments were performed for two types of buildings, three-floor family houses and five-floor apartment buildings. The atmospheric boundary layer (ABL) models were generated by means of the Counihan method using a castellated barrier wall, vortex generators and a fetch of roughness elements. A hot wire anemometry system was applied for measurement of mean velocity and velocity fluctuations. The mean velocity profiles are in good agreement with the power law for exponent values from α = 0.15 to α = 0.24, which is acceptable for the representation of the rural and suburban ABL, respectively. Effects of the spacing density among buildings on wind characteristics range from the ground up to 0.6δ . As the spacing becomes smaller, the mean flow is slowed down, whilst, simultaneously, the turbulence intensity and absolute values of the Reynolds stress increase due to the increased friction between the surface and the air flow. This results in a higher ventilation efficiency as the increased retardation of horizontal flow simultaneously accompanies an intensified vertical transfer of momentum.

Construction and experimental testing of the constant-bandwidth constant-temperature anemometer

A classical constant-temperature hot-wire anemometer enables the measurement of fast-changing flow velocity fluctuations, although its transmission bandwidth is a function of measured velocity. This may be a source of significant dynamic errors. Incorporation of an adaptive controller into the constant-temperature system results in hot-wire anemometer operating with a constant transmission bandwidth. The construction together with the results of experimental testing of a constant-bandwidth hot-wire anemometer prototype are presented in this article. During the testing, an approximately constant transmission bandwidth of the anemometer was achieved. The constant-bandwidth hot-wire anemometer can be used in measurements of high-frequency variable flows characterized by a wide range of velocity changes.

Intermittency and rough-pipe turbulence

Recently, by analyzing the measurement data of Nikuradze [NACA Tech. Memo No. 1292 (1950)], it has been proposed [N. Goldenfeld, Phys. Rev. Lett. 96, 044503 (2006)] that the friction factor, f , of rough-pipe flow obeys a scaling law in the turbulent regime. Here, we provide a phenomenological scaling argument to explain this law and demonstrate how intermittency modifies the scaling form, thereby relating f to the intermittency exponent, eta . By statistically analyzing the measurement data of f , we infer a satisfactory estimate for eta ( approximately 0.02) , the inclusion of which is shown to improve the data-collapse curve. This provides empirical evidence for intermittency other than the direct measurement of velocity fluctuations.

Unsteady separated and reattaching turbulent flow over a two-dimensional square rib

Abstract The spatio-temporal characteristics of the separated and reattaching turbulent flow over a two-dimensional square rib were studied experimentally. Synchronized measurements of wall-pressure fluctuations and velocity fluctuations were made using a microphone array and a split-fiber film, respectively. Profiles of time-averaged streamwise velocity and wall-pressure fluctuations showed that the shear layer separated from the leading edge of the rib sweeps past the rib and directly reattaches on the bottom wall (x/H=9.75) downstream of the rib. A thin region of reverse flow was formed above the rib. The shedding large-scale vortical structures (fH/U0=0.03) and the flapping separation bubble (fH/U0=0.0075) could be discerned in the wall-pressure spectra. A multi-resolution analysis based on the maximum overlap discrete wavelet transform (MODWT) was performed to extract the intermittent events associated with the shedding large-scale vortical structures and the flapping separation bubble. The convective dynamics of the large-scale vortical structures were analyzed in terms of the autocorrelation of the continuous wavelet-transformed wall pressure, cross-correlation of the wall-pressure fluctuations, and the cross-correlation between the wall pressure at the time-averaged reattachment point and the streamwise velocity field. The convection speeds of the large-scale vortical structures before and after the reattachment point were Uc=0.35U0 and 0.45U0, respectively. The flapping motion of the separation bubble was analyzed in terms of the conditionally averaged reverse-flow intermittency near the wall region. The instantaneous reattachment point in response to the flapping motion was obtained; these findings established that the reattachment zone was a 1.2H-long region centered at x/H=9.75. The reverse-flow intermittency in one period of the flapping motion demonstrated that the thin reverse flow above the rib is influenced by the flapping motion of the separation bubble behind the rib.

On the high contraction ratio anomaly of axisymmetric contraction of grid-generated turbulence

The present paper deals with the anomalous increase in second-order moments of longitudinal velocity fluctuations reported in experiments carried out to measure properties of axisymmetric strained turbulence when the strain is provided by axisymmetric contractions (nozzles). Experimental evidence is provided that the increase is due to inaccuracies in the measurements. It is shown that the imperfect spatial resolution of X-wire probes, the mass flow rate fluctuations in the flow facility, and the electronic noise contaminate the hot-wire measurements of not only the longitudinal but also the transverse velocity fluctuation components. These kinds of contaminations start to dominate the measurements, especially when the contraction ratio of the nozzle is high and/or the turbulence level is low and turbulence length scales are small. A series of measurement and correction methods is proposed employing two single normal wires and one inclined wire to detect and eliminate all three types of contaminations of longitudinal and transverse velocity fluctuation measurements. These methods are subsequently employed to carry out turbulence measurements of grid turbulence strained in a converging nozzle with a high contraction ratio. The resulting Reynolds stress developments along the nozzle do not involve an erroneous increase in longitudinal turbulent velocity fluctuations and are consistent with theoretical expectations such as the direct numerical simulations and the rapid distortion theory. The corrected results revealed that turbulence along the nozzle reaches the two-component axisymmetric state when looked at through the anisotropy of Reynolds stresses. Furthermore, the anisotropy of Reynolds stresses exhibits zero gradient at that state. For the first time in the literature, experimental confirmation of rapid distortion theory is supplied for the axisymmetric contraction of turbulence within nozzles having a contraction ratio higher than 9. Finally, a set of experimental data is provided for the validation of turbulence models.

Flow Around a Circular Cylinder with Tangential Blowing near a Plane Boundary (2nd Report, A Study on Unsteady Characteristics)

The wake oscillation of a circular cylinder, which is placed near a rigid plane boundary, with tangential blowing from a surface slot is investigated experimentally under the condition of Re=2.7 ×104. The time-mean surface pressure measurements on the circular cylinder, flow visualizations and velocity fluctuation measurements were carried out for various momentum coefficient Cμ and clearances between the circular cylinder and the rigid plane boundary changed. Based on these experiments it is found that periodic velocity fluctuation, which is associated with vortex shedding (Karman vortices), is suppressed by the tangential blowing. However, in the case of the cylinder placed near the rigid plane boundary, the reverse flow occurs in the clearance between the cylinder and the boundary, in particular Cμ increases. The level of flow unsteadiness naturally increases. Moreover, velocity fluctuation measurements revealed that the velocity fluctuation classified into two cases, one being the velocity fluctuation occurring Periodically (H/D>0.1) and the other occurring non-periodically (H/D=0.1).

Intermittent Bursting of Turbulence in a Stable Boundary Layer with Low-level Jet

The atmospheric stable boundary layer (SBL) with a low-level jet is simulated experimentally using a thermally stratified wind tunnel. The turbulence structure and flow characteristics are investigated by simultaneous measurements of velocity and temperature fluctuations and by flow visualization. Attention is focused on the effect of strong wind shear due to a low-level jet on stratified boundary layers with strong stability. Occasional bursting of turbulence in the lower portion of the boundary layer can be found in the SBL with strong stability. This bursting originates aloft away from the surface and transports fluid with relatively low velocity and temperature upward and fluid with relatively high velocity and temperature downward. Furthermore, the relationship between the occurrence of turbulence bursting and the local gradient Richardson number (Ri) is investigated. The Ri becomes larger than the critical Ri, Ricr = 0.25, in quiescent periods. On the other hand, the Ri number becomes smaller than Ricr during bursting events.

Constant-bandwidth constant-temperature hot-wire anemometer

A constant-temperature anemometer (CTA) enables the measurement of fast-changing velocity fluctuations. In the classical solution of CTA, the transmission band is a function of flow velocity. This is a minor drawback when the mean flow velocity does not significantly change, though it might lead to dynamic errors when flow velocity varies over a considerable range. A modification is outlined, whereby an adaptive controller is incorporated in the CTA system such that the anemometer's transmission band remains constant in the function of flow velocity. For that purpose, a second feedback loop is provided, and the output signal from the anemometer will regulate the controller's parameters such that the transmission bandwidth remains constant. The mathematical model of a CTA that has been developed and model testing data allow a through evaluation of the proposed solution. A modified anemometer can be used in measurements of high-frequency variable flows in a wide range of velocities. The proposed modification allows the minimization of dynamic measurement errors.

Experimental and numerical investigation of counter-current stratified flows in horizontal channels

Counter-current flow regimes of air and water are investigated in the WENKA test facility at the Forschungszentrum Karlsruhe. With the fluorescent-particle image velocimetry (PIV) measurement technique, velocity and velocity fluctuations are measured up to the free surface. A statistical model is presented to correlate the measured void fraction with the turbulent kinetic energy calculated from the measured velocity fluctuations. The experimental data are used to develop a phase interaction model to simulate stratified flows. Two different approaches are compared for turbulence modelling. The Prandtl mixing length model and an extended k– ω model for the two-phase region are applied to supercritical flow conditions.

Measurements of premixed-flame turbulence generation and modification in a Taylor–Couette burner

Turbulent, premixed lean methane–air flames were studied experimentally in a Taylor–Couette burner, extending the previous work of non-reacting turbulent-flow measurements. A laser-Doppler velocimetry system is employed to measure velocity fluctuations in the circumferential direction at the center of the annulus where mean velocities are nearly zero. Turbulence parameters such as the intensities, approximated integral and micro-time and length scales and one-dimensional frequency spectra are obtained for the flow-field ahead and behind the flame front. The frequency spectra exhibit a −5/3 slope reaffirming isotropic characteristics. It is found that there is an increase in intensity, turbulence Reynolds number and energy across a broad range of frequencies behind the flame along with a shift toward larger scales. However, there appears to be a decrease in amplification of the intensities and turbulence Reynolds number with increasing pre-ignition turbulence in the burner (generated by counter-rotation of the cylinder walls). Results suggest that the presence of flame-generated turbulence in the TC burner is sensitive to both pre-ignition turbulence and equivalence ratio.

A wall-bounded turbulent mixing layer flow over an open step: part 2. Unsteady characteristics

The unsteady characteristics of the wall-bounded turbulent mixing layer flow over an open step were investigated. Simultaneous measurements of wall pressure fluctuations and velocity fluctuations were accomplished using a microphone array and X-type gold-plated hotwire, respectively. To elucidate the scale-variant flow structure superimposed on the wall-bounded turbulent mixing layer flow, a multi-resolution analysis based on the maximum overlap discrete wavelet transform (MODWT) was performed. The large-scale vortical structure shedding at the frequency St = 0.075 was extracted using the autocorrelation of the continuous wavelet transformed wall pressure. The convective features of the vortices at St = 0.075 and the higher harmonic frequencies St = 0.15 and 0.29 were delineated in terms of the conditionally averaged velocity field and Reynolds stress field, the scale-variant cross-correlation of the wall pressure field, and the cross-correlation between the conditionally averaged wall pressure and the velocity field. The flow rate of fluid entrained through the open step was the highest when the large-scale vortical structure came into contact with the wall at x/H = 15.0.

Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High-intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10–12% and an integral length scale of 2cm(Λx∕c=0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder, turbine vane, and turbine blade) and boundary layer conditions.

Experimental Investigation of Reynolds and Favre Averaging in High-Speed Jets

Recent advancements in a molecular Rayleigh scattering based diagnostic technique allowed for simultaneous measurement of velocity and density fluctuations with high sampling rates. The technique was used to investigate unheated high subsonic and supersonic fully expanded free jets in the Mach number range from 0.8 to 1.8. The difference between the Favre-averaged and the Reynolds-averaged axial velocity and axial component of the turbulent kinetic energy is found to be small. On average, estimates based on Morkovin’s strong Reynolds analogy are found to underpredict turbulent density fluctuations.

A method for the redesign of pneumatic dryers

A method for the design and redesign of pneumatic dryers is proposed. As an example, this pragmatic and fast method is applied to the redesign of a pneumatic dryer for potato particles that suffered from ignition of resident particles. In the method, a duct, geometrically qua flow dynamics equivalent to the industrial installation, as well as analysis-based modifications of it, are studied using velocity fluctuation measurements. For most practical flows of gas-particles, one-way coupling usually suffices to assess the influence of the flow on particle migration and deposition. Redesign proposals are based on measured fluctuation levels. The typical example given concerns optimization of the inclination of a ramp, air inlet and a grid. Some of these recommendations have successfully been applied in practice.

Hot wire anemometry for the measurement of temperature and velocity fluctuations inside a thermoacoustic cooler

Hot wire anemometry was used to measure velocity and temperature fluctuations inside a thermoacoustic refrigerator. In an oscillating flow with zero-mean velocity, such as in an acoustic standing wave, heat transfer is different from heat transfer in a steady flow. A dynamic calibration of the system is required. This calibration was done inside the resonator of the cooler, with no stack and no heat exchangers. Two thermoacoustic systems were studied to calibrate the anemometer in air at atmospheric pressure and in He/Ar mixtures at a mean pressure of 6 bars. Measurements of temperature and velocity were done at the end of the stack and heat exchangers. Correlations between temperature and velocity fluctuations were obtained, and the enthalpy convective flux was computed. These results were compared with results obtained from numerical simulations recently done at Ecole Centrale de Lyon [D. Marx, AIAA J. 42(7) (2004)].

The Fluid Dynamical Properties of a Dilute Surfactant Solution around a Circular Cylinder

Influence of adding cationic surfactant (Ethoquard O12) with a consentration of 50 ppm on water flow around a cylinder in a range of Reynolds number Res between from 64 to 600 was investigated by using LDV and PIV. At Res =300, the flow with the surfactant formed a stagnant zone, which appeared periodically, in front of the cylinder expanding along the axial direction. The period of the appearance was about 120 [s]. The periodic flow structure was visualized by the measurements of velocity fluctuation. The development of the stagnant zone in front of the cylinder induced the expansion of a low speed flow area near the side of the cylinder.

Unsteady Off-Design Velocity and ReynoldsStresses in an Axial Compressor

The aim of the present work is to provide unsteady experimental data about the flow in a one-stage axial compressor. Some particular flow features are described and discussed. The operating conditions considered here concern an off-design point that is dose to the compressor rotating stall limit. Measurements were performed with a three-dimensional laser Doppler velocimeter. Twenty-eight axial sections were surveyed from the rotor inlet to the stator outlet. In each section, velocity and velocity fluctuation measurements concerned more than 250 points. Thus, more than 8000 locations of measurement were investigated in the rotating and in the fixed parts of the stage. The main flow patterns associated with the rotor unsteadiness are described through a few pictures of the measured velocity components and of the Reynolds-stress distributions. Obviously, the most outstanding flow phenomena occur in the upper part of the rotor channels and take place along the stator blades suction side. In the moving row, secondary complex flow structures are highlighted, and the structure of the separated flow along the stator blade is analyzed.

Effect of initial conditions on decaying grid turbulence at low R λ

The transport equations for the second-order velocity structure functions 〈(δu)2〉 and 〈(δq)2〉 are used as a scale-by-scale budget to quantify the effect of initial conditions at low Reynolds numbers, typical of grid turbulence. The validity of these equations is first investigated via hot-wire measurements of velocity and transverse vorticity fluctuations. The transport equation for 〈(δq)2〉 is shown to be balanced at all scales, while anisotropy of the large scales leads to a significant imbalance in the equation for 〈(δu)2〉. The effect of using similarity to evaluate the transport equation is rigorously tested. This approach has the desirable benefit of requiring less extensive measurements to calculate the inhomogeneous term of the transport equation. The similarity form of the 〈(δq)2〉 equation produces nearly identical results as those obtained without the similarity assumption. In the case of the 〈(δu)2〉 equation, the similarity method forces a balance at large separation, although the imbalance due to large scale anisotropy remains. The initial conditions of the turbulence at constant RM ≃ 10,400 (28≤ Rλ≤ 55) are changed by using three grids of different geometries. Initial conditions affect the shape and magnitude of the second- and third-order structure functions, as well as the anisotropy of the large scales. The effect of initial conditions on the scale-by-scale budget is restricted to the inhomogeneous term of the transport equations, while the dissipation term remains unaffected despite the low Rλ. Scales as small as λ are affected by the changes in initial conditions.