Sergio Viçosa Möller

On phenomena of turbulent flow through rod bundles

Abstract Experimental studies have shown that the axial and azimuthal turbulence intensities in the gap regions of rod bundles increase strongly with decreasing gap spacing; the fluctuating velocities in the axial and azimuthal directions have a quasi-periodic behavior. To determine the origin of this phenomenon and its characteristics as a function of the geometry and the Reynolds number, an experimental investigation was performed on the turbulent flow in several rod bundles with different aspect ratios. Hot wires and microphones were used for the measurements of velocity and wall pressure fluctuations. The data were evaluated to obtain spectra as well as autocorrelations and cross correlations. Based on the results, a phenomenological model is proposed to explain this phenomenon. By means of the model, the mass exchange between neighboring subchannels is explained.

Single-phase turbulent mixing in rod bundles

In the subchannel analysis of rod bundles, the mass exchange between adjacent subchannels is quantified by mixing coefficients, or mixing factors. This process of turbulent cross-flow mixing was found to be related to the phenomenon of quasi-periodic “flow pulsations” across the gaps between adjacent subchannels. By means of hot-wire measurements of spectra of the components of the turbulent fluctuating velocities in the gap regions and a phenomenological model for the flow pulsations, mixing factors were determined for several four-rod bundles with different aspect ratios (P/D, W/D). Comparison of the results with the literature shows good agreement. A correlation for the mixing factor as a function of the dimensionless gap width, obtained by linear regression of the new data, is proposed.

Characteristics of the low-speed wind tunnel of the UNNE

Abstract This paper presents the evaluation of the characteristics of the open-loop low-speed UNNE Wind Tunnel to verify its applicability to similarity studies and to experimental simulations of the atmospheric boundary layer. For this purpose a hot wire anemometry system was implemented for the measurements of mean velocity and velocity fluctuations. Data acquisition was performed by means of an A/D converter board connected to a personal computer. Experimental results are presented in form of velocity profiles and turbulence intensities as well as power spectral distributions of the axial component of the velocity fluctuations. Results of measurements in the empty tunnel showed a uniform velocity field and low turbulence intensities. Analysis of atmospheric boundary-layer simulations by means of Counihan and Standen methods showed the adequecy of the tunnel for natural wind simulations.

On the fluctuating wall pressure field in tube banks

This paper presents the experimental analysis of pressure and velocity fluctuations of the cross flow in tube banks, with triangular and square arrangements, and four different aspect ratios. Air is the working fluid, driven by a centrifugal blower, passed by a settling chamber and a set of honeycombs and screens, before reaching the tube bank at an incidence angle of 90°. Both triangular and square arrangements have pitch-to-diameter ratios P/D=1.60, 1.26, 1.16 and 1.05, with Reynolds numbers, calculated with the tube diameter and the velocity of the flow in the narrow gap between the tubes, from Re=4×104 to 7×104. Velocity and velocity fluctuations were measured by a constant-temperature hot-wire anemometer, while pressure fluctuations were measured by a piezo-resistive pressure transducer mounted inside one of the tubes in the bank. Behavior of fluctuating quantities is described by means of dimensionless root mean square values and autospectral density functions, while their interdependence is discussed based on cross-correlation functions.

Numeric and experimental analysis of the turbulent flow through a channel with baffle plates

The present work presents the numeric and experimental analysis of the turbulent flow of air inside a channel of rectangular section, containing two rectangular baffle plates. This is an important problem in the scope of heat exchangers where the characterization of the flow, pressure distribution, as well as the existence and the extension of possible recirculations need to be identified. The differential equations that describe the flow were integrated by the Finite Volumes Method, in two dimensions, employing the Fluent software with the k-e model to describe the turbulence. The mesh is structured, with rectangular volumes. Several boundary conditions were explored, being the more realistic results obtained by prescribing the inlet velocity field and atmospheric pressure at the exit. The obtained results are compared with experimental data, being analyzed and commented the deviations. The velocity field was measured with a hot wire anemometer, and the pressure field with an electronic manometer. The largest variations in the pressure and velocity fields occur in the regions near to the deflectors, as expected.

Experimental Study of the Bistable Flow in Tube Arrays

Flow through circular cylinder arrays are commonly found in several engineering application as offshore structures, heat exchangers, transmission lines and chimneys, therefore the understanding of the several phenomena that occur due the interaction between flowing fluid and these structures is very important. This work analyzes experimentally the presence of phenomenon called bistable or biased flow, which is able to cause alternations on the flow modes in cylinder arrays. For the experimental work, two cylinders are placed side-by-side, or forming a tube bank with in line arrangement. The first arrangement corresponds to the case where the phenomenon is easily detected, and in line tube bank, the phenomenon has been also found. The experiments were performed in a wind channel using hot wire anemometry additionally to the classical statistic and spectral tools, wavelet transforms are used. The bistable flow can be an important generator of dynamic instabilities, since it alternates the lift and drag coefficients, thus alternating the structure dynamic response.

Velocity and pressure fluctuations on inclined tube banks submitted to turbulent flow

This paper presents the experimental study of pressure and velocity fluctuations and their interdependence, in the turbulent flow impinging on arrangements of yawed circular cylinders simulating inclined tube banks with square arrangement and a pitch to diameter ratio of 1.26. Measurements were performed with hot-wires and a pressure transducer. Behaviour of fluctuating quantities is described by means of dimensionless autospectral density functions and cross correlations. Experimental results indicates the presence of two different phenomena of vortex generation in inclined tube banks.

Symbolic Dynamics Applied to the Identification of Flow Patterns Inside Tube Banks

Abstract This paper presents a study of the identification of flow patterns inside a tube bank with the technique of symbolic dynamics. The experimental signals of the mean velocity and its fluctuations are measured by hot-wire anemometry in an aerodynamic channel and used as input data for the symbolic dynamics technique. The tube bank consists of 23 circular cylinders in a triangular arrangement. The pitch-to-diameter ratio chosen was 1.26 and the Reynolds numbers are in the range from 7.5 × 103 to 4.4 × 104, computed with the tube diameter, D = 25.1 mm, and the percolation velocity. In this work, a binary alphabet was chosen to convert and analyze the data. The partitioning process is performed through the mean value of the time series and via discrete wavelet reconstruction, according to a chosen reconstruction level. The flow patterns are presented for different positions inside the tube bank, where histograms and probability density functions support the statistical interpretation. The histograms with a decimal representation for the original experimental time series with partitioning performed through the mean value show that the signals do not present fast changes of velocity fluctuations. This behavior was observed in the five rows of cylinders. However, by changing the partitioning according to a wavelet reconstruction of the signal with high frequency, which means that the signals are close to the partitioning function, fast changes appear in all of the time series observed. The results indicate that the turbulence in tube banks has chaotic characteristics. Flow visualizations performed with ink injection inside the tube bank helped in the interpretation of the results.

Experimental study of the propagation of a far-field disturbance in the turbulent flow through square array tube banks

Summary and Discussion Experimental results in this paper show that a disturbance generated upstream of the investigated tube banks is amplified as it passes the first row of tubes. The disturbance in this work is a wake from a vortex generator. The frequencies of the disturbance were carefully chosen to avoid being confused with the frequencies generated in the bank, vibrations of the test section or due to acoustic resonances. Results of spectra of velocity fluctuations show the presence of peaks from the disturbance in the wake frequency until the 4th or 5th tube rows. The values of the spectra measured in the wider region between the first and the second row have higher values than the original disturbance. This raise in the amplitude of the disturbance is accompanied by the growth of the turbulence intensity. After passing the second row, turbulence levels remain with almost constant values as the flow passes through the successive rows, while the disturbance vanishes, in some cases only after the 4th or the 5th tube row.

EXPERIMENTAL ANALYSIS OF TURBULENT TRANSVERSAL FLOW ON TWO FIXED PARALLEL CYLINDERS WITH OSCILLATORY AND ROTATIONAL FREEDOM

This paper presents an experimental study of the bistability phenomenon that occurs in the simplified geometry of two tubes arranged side by side submitted to a turbulent cross flow. In the analysis we consider the condition of fixed tubes and with a degree of freedom (rotational), submitted to different numbers of Reynolds. The experimental technique consists in the measurement of flow rate fluctuations through the aerodynamic channel hot wire anemometry and the evaluation of the stresses of the flow on the tubes with the use of a load cell. The data obtained from the measurement in the channel are treated with the use of statistical, spectral and wavelet tools. In the experimental results we observe the presence of bistability and note the non-simultaneous behavior of the phenomenon along the cylinders. The asymmetric formation of the wakes is also studied and attributed to the interaction between the vortices from wakes behind the cylinders, since, initially, the wake detachments is symmetrical. In the evaluation of the forces by means of the extensiometry technique, the steps of conditioning and acquisition of the signal were elaborated and the results of mechanical deformation presented similarity with the phenomenon observed in the experimental anemometry. For a Reynolds number of 27635 a force of 26.1 mN was found with a torque developed in the load cell of 84.95 Nmm.