A novel type of semi-active jet turbulence grid

Abstract

This article describes a novel approach to generate increased turbulence levels in an incoming flow. It relies on a cost-effective and robust semi-active jet grid, equipped with flexible tubes as moving elements attached onto tube connections placed at the intersections of a fixed, regular grid. For the present study, these flexible tubes are oriented in counter-flow direction in a wind tunnel. Tube motion is governed by multiple interactions between the main flow and the jets exiting the tubes, resulting in chaotic velocity fluctuations and high turbulence intensities in the test section. After describing the structure of the turbulence generator, the turbulent properties of the airflow downstream of the grid in both passive and active modes are measured by hot-wire anemometry and compared with one another. When activating the turbulence generator, turbulence intensity, turbulent kinetic energy, and the Taylor Reynolds number are noticeably increased in comparison with the passive mode (corresponding to simple grid turbulence). Furthermore, the inertial subrange of the turbulent energy spectrum becomes wider and closely follows Kolmogorov's -5/3 law. These results show that the semi-active grid, in contrast to passive systems, is capable of producing high turbulence levels, even at low incoming flow velocity. Compared to alternatives based on actuators driven by servo-motors, the production and operation costs of the semi-active grid are very moderate and its robustness is much higher.

Keywords: Mechanical engineering.

Fig. 1

Experimental setup with a zoom on the test section, including the reference system as well as measurement points and lines discussed later in the text.

Fig. 2

(a) semi-active jet grid before starting the operation; (b) flexible tube elements and closed orifices shown for an enlarged view of part of the grid.

Fig. 3

Trajectories of two neighboring tube ends during a time period of 3.2 seconds, normalized by M. Left trajectory (continuous line) was plotted from the tube origin (0, 0), while the dashed line trajectory was plotted from (2M, 0), as in reality.

Fig. 4

(a) Isotropy ratio versus distance downstream of the grid. Passive mode: open markers; semi-active mode: filled markers (triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s). (b) Profiles of isotropy ratio i in semi-active mode at x/M = 20 for U_ref = 4 m/s (crosses: horizontal measurement line; circles: vertical measurement line).

Fig. 5

Homogeneity profile measured in the vertical direction at U_ref = 4 m/s. Passive mode: open markers; semi-active mode: filled markers. Squares: x/M = 20; Circles: x/M = 50.

Fig. 6

(a) Decay of turbulence intensity along the centerline of the test section. Passive mode: open markers; semi-active mode: filled markers; line: empirical curve T_u = 1.13(x/d)^−5/7 from Roach (1987). (b) Ratio of turbulence intensity between semi-active and passive modes. In both cases, the mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s.

Fig. 7

(a) Evolution of the kinetic energy of turbulence along the centerline (log-log scale). Passive mode: open markers; semi-active mode: filled markers. (b) Ratio of turbulent kinetic energy between semi-active and passive mode. In both cases, the mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s.

Fig. 8

Evolution of the turbulent kinetic energy dissipation rate ε along the centerline of the test section (log-log scale). Passive mode: open markers; semi-active mode: filled markers. The mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s.

Fig. 9

Evolution of the dissipation constant C_ε along the centerline of the test section (log-log scale). Passive mode: open markers; semi-active mode: filled markers. The mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s.

Fig. 10

Kolmogorov microscale as a function of normalized downstream distance from the grid. Passive mode: open markers; semi-active mode: filled markers. The mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s.

Fig. 11

(a) Taylor Reynolds number as a function of normalized distance from the grid. Passive mode: open markers; semi-active mode: filled markers. (b) Ratio of Taylor Reynolds number between semi-active and passive mode. In both cases, the mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s.

Fig. 12

(a) Longitudinal one-dimensional energy spectra at x/M = 30. Passive mode: open markers; semi-active mode: filled markers. In both cases, the mean inflow velocity was varied as follows: triangles: U_ref = 3 m/s; diamonds: U_ref = 4 m/s; pentagrams: U_ref = 5 m/s; hexagrams: U_ref = 6 m/s. (b) Normalized longitudinal energy spectra for U_ref = 6 m/s at x/M = 30 (plus signs), 40 (circles), and 50 (crosses). Passive mode: continuous lines; semi-active mode: dashed lines. The straight solid lines correspond to the -5/3 power law. The inset shows the compensated spectra.