A Study on the Decay of Grid Turbulence and Its Statistics using Hotwire Anemometry (original) (raw)

Measurement of Turbulence Statistics Using Hot Wire Anemometry

The higher order statistical moments of the turbulence signal are essential for finding the quality of the flow at any given location. In the present study, these moments are used for characterizing the flow field at the test section of low speed wind tunnel in CSIR-CMERI. The fluctuating velocity inside the wind tunnel is measured using a multi channel hot wire anemometry system. The calibration of probes and the effect of overheat ratios of constant temperature anemometry system on the sensitivity of velocity, turbulence measurements are discussed in detail. The higher order moments, e.g., mean, variance, skewness and flatness factor, of fluctuating velocity and the turbulence intensity are calculated at various location along the lateral direction of the test section. It has been found that the higher order moments are highly essential in characterizing the flow quality at the test section besides turbulence intensity

Experimental Investigations on Low Speed Turbulence Using Hot-Wire Anemometer

2013

Objective of this study is to measure the turbulence characteristics of air flow through rectangular channel in the developing regime. IFA 300 Constant Temperature Anemometer system is used for turbulence measurements. Calibration of IFA 300 Constant Temperature Anemometer is carried out in a low speed open jet wind tunnel using known velocity of air. Calibration constants are determined as per the King's Law. Measurements are carried out at different Reynolds Numbers for the analysis of variation of velocity from wall to core region. The turbulence measurement includes variation of turbulent intensity and turbulence kinetic energy for different Reynolds Numbers. The results obtained from hot wire anemometer are compared with analytical methods available in literature. Fast Fourier Transform is also carried out for establishing the chaotic nature of turbulent flow. Turbulence intensity, turbulence kinetic energy and normal stress are found to decrease from wall to core region of rectangular channel.

Effects of initial conditions in decaying turbulence generated by passive grids

Journal of Fluid Mechanics, 2007

The effects of initial conditions on grid turbulence are investigated for low to moderate Reynolds numbers. Four grid geometries are used to yield variations in initial conditions and a secondary contraction is introduced to improve the isotropy of the turbulence. The hot-wire measurements, believed to be the most detailed to date for this flow, indicate that initial conditions have a persistent impact on the large-scale organization of the flow over the length of the tunnel. The power-law coefficients, determined via an improved method, also depend on the initial conditions. For example, the power-law exponent m is affected by the various levels of large-scale organization and anisotropy generated by the different grids and the shape of the energy spectrum at low wavenumbers. However, the results show that these effects are primarily related to deviations between the turbulence produced in the wind tunnel and true decaying homogenous isotropic turbulence (HIT). Indeed, when isotrop...

Procedure for Determining Turbulence Length Scales Using Hotwire Anemometry

2014

Hotwire anemometers are used to measure instantaneous velocity from which the mean velocity and the velocity fluctuation can be determined. Using a hotwire system, it is possible to deduce not only the velocity components and their fluctuation but to also analyze the energy spectra and from that the turbulence length scales. In this experiment, hotwire anemometry is used to measure the flow field turbulence for an array of film cooling holes. The objective of this paper is to document the procedure that is used to reduce the instantaneous velocity measurements to determine the turbulence length scales using data from the film-cooling experiments to illustrate the procedure.

Characterization of Low Turbulence Wind Tunnel

2006

Wind tunnels with uniform velocity profiles and low turbulence are used to calibrate air velocity sensors such as anemometers and Pitot tubes among others. These wind velocity sensors are used in cases such as, for example, eolic turbines towers, meteorological stations, hospitals, etc. To make calibrations with high precision and accuracy it is necessary a wind tunnel with low turbulence (less than 0.4%) and uniform velocity profile in the test section. This study presents a characterization of the low turbulence wind tunnel of the Anemometry Laboratory of IPT (Institute for Technological Research), in which the calibrations are done in the discharge of the wind tunnel and the working range is from 2 m/s up to 40 m/s. A turbulence intensity less than 0.4% and a mean velocity variation of ± 0.2 % was obtained.

Measurement of Turbulence in Wind Tunnel with Screens

Acta Avionica Journal

Turbulence in wind tunnel is measured by two methods – by the measurement of aerodynamic drag of sphere and by the hot-wire CTA measurement of turbulent fluctuations. Measurements are compared with the prediction of turbulence for various configurations of screens.

The decay power law in grid-generated turbulence

Journal of Fluid Mechanics, 1990

The effect of initial conditions on the decay exponent and coefficient and virtual origin in the decay power-law form for the variation of the variance of the turbulent velocity downstream of biplane grids constructed of rods of both round and square cross-section is determined. This effect is determined for data obtained as part of the present study as well as from previous studies. These studies cover a Reynolds number range from 6000 to 68000, mesh sizes of 2.54 and 5.08 cm, and solidities of 0.34 and 0.44. It is shown that the choice of the virtual origin and the use of data in the nonhomogeneous portion of the flow can have a significant influence on the value of the parameters in the decay power-law. Criteria are developed to identify the nearly homogeneous and isotropic portion of the flow. These criteria include low values of the velocity skewness, constancy of the skewness of the velocity derivative and balance of the turbulent kinetic energy equation. Results based on data selected by means of these criteria show that the decay exponent and virtual origin are independent of initial conditions such as Reynolds number, mesh size, solidity, and rod shape and surface roughness with values of respectively 1.30 and 0. In contrast and as expected, the decay coefficient is found to be a function of these initial conditions. Thus, the downstream variation of the variance of the turbulent velocity is universally self-similar.

Determination of Turbulence Level in the TA-2 Aerodynamic Wind Tunnel

2003

An experimental investigation has been carried out at the TA-2 Aerodynamic Wind Tunnel test section of the Institute of Aeronautics and Space – IAE at Aerospace Technical Center – CTA. Preliminary results of turbulence intensity and velocity measurements were determined by using a computer-controlled constant-temperature anemometer system. Such results are presented in this paper and compared to previous turbulence sphere results. The main purpose of this work is to examine the behavior of the flow at TA-2 through the development and application of this technique, which will be integrated to other advanced technologies to give support to the aerodynamic tests carried out at this wind tunnel. Good agreements between velocity profiles measured with hot wire-anemometer and with pitot-static rake were observed. It was also observed that the turbulence intensity at TA-2 is low.

The Decay of Turbulence Generated by Square Grid and Spaced Square Grid

Topical Problems of Fluid Mechanics 2022, 2022

Fractal grids generate multi-scale turbulent flows which interact with each other and form new multi-scales with different properties compared with those generated by classical grids. The goal of this work was to study the influence of geometry multiscale fractal on the generation and decay of turbulence by comparing the turbulence produced by a traditional fractal square grid FSG for short to that produced by a spaced fractal square grid SFSG for short with similar physical properties. The velocity measurements were performed in an open circuit suction type wind tunnel using a constant temperature hot wire anemometer at various positions in the x direction along the tunnel’s center line and for three different Reynolds numbers ReL0 based on the inlet velocity U? and the length of the largest grid bar L0. It was found that the turbulence intensity decays with distance from the grid for SFSG. Whereas, for FSG at low Reynolds numbers the turbulence intensity shows the same behaviour a...