cases:case075

# Flow through Axisymmetric Expansions

This study examines the flow field in three axisymmetric expansions having diffuser half-angles, $\alpha$, of 14, 18, and 90 degrees, respectively, as shown in figure 1. Velocity measurements were performed at a Reynolds number of $Re = 1.56 \times 10^4$ using a single component LDA operated in forward scatter mode. The test facility was refractive index matched, allowing measurement of the velocities $U$, $V$, $W$, and Reynolds stresses $\overline{u^2}$, $\overline{v^2}$, $\overline{w^2}$, $\overline{uv}$ and $\overline{uw}$ upstream of, and throughout, the entire recirculation region.

Fig. 1: Flow geometry

#### Flow Facility

Experiments were conducted in the vertically mounted refractive index matched test facility . The flow was drawn from a constant head tank through a nozzle and flow straighteners into an inlet pipe of diameter 50 mm ($D_1= 2 R_1$). A trip wire was placed immediately downstream of the inlet nozzle to ensure a fully developed turbulent pipe flow at the inlet to the test section 30 diameters downstream.

The test section consisted of interchangeable diffuser sections having half-angles of 14, 18, or 90 degrees. The downstream diameter was 80 mm ($D_2=2R_2$), resulting in a diameter ratio $D_2/D_1$ of 1.6 and an area ratio of 2.56. The maximum achievable Reynolds number based on the mean flow velocity was $Re = 1.56 \times 10^4$. The centreline velocity upstream of the diffuser, $U_o$, was used as a normalising velocity in the presentation of results and was equal to 2.51 m/s for all geometries.

The test section was encased in an outer, square-sided containment vessel extending l00mm upstream and 600mm downstream of the diffuser inlet. The volume between the test section and the outer containment was filled with the working fluid, which was selected to have a refractive index equal to that of the containment glass. Thus the test section pipe was mechanically present but optically transparent, facilitating the use of the laser Doppler anemometer for velocity measurements. The match of refractive indices was achieved by mixing two diesel oils with different refractive indexes in proportions suitable to yield the desired index of refraction. Since the refractive index is dependent on temperature, a heat exchanger was incorporated into the test loop to regulate the temperature to within +/-0.5oC. Further details of all aspects of the experimental facility are described by Wieser and Nitsche (1987, 1988).

#### Laser Doppler Anemometer

The LDA was a one-component, forward scatter system operating with a He-Ne I laser.

#### Measurement Errors

The absolute positioning accuracy in three directions are estimated to be: $z$: $\pm 100\mu$m; $r$ (perpendicular to optical axis): $\pm 100\mu$m; $r$ (parallel to optical axis) $\pm 1$ mm. The radial positioning parallel to the optical axis was less accurate due to the 2 mm length of the control volume.

Data available include profiles of mean velocity components and Reynolds stresses at a number of streamwise locations, ranging from 30 mm upstream of the expansion corner to 200 mm downstream of it.

Sample plots of selected quantities are available.

The file readme.txt contains some description of the files.

1. Stieglmeier, M., Tropea, C., Weiser, N., Nitsche, W. (1989). Experimental investigation of the flow through axisymmetric expansions. J. Fluids Engineering, Vol. 111, pp. 464-471.
2. Weiser, N., Nitsche, W. (1987). Experimentelle und numerische Untersuchungen zur abgelösten Strömung an einer Rohrstufe mit variablem Öffnungswinkel, ILR-Mitteilung 177, Institut für Luft- und Raumfahrt, Berlin.
3. Weiser, N., Nitsche, W. (1988), Zur rotationssymmetrischen Innenströmung mit Ablosung, ILR-Mitteilung 193, Institut für Luft- und Raumfahrt, Berlin.

Indexed data:

case075 (dbcase, confined_flow)
case075
titleFlow through axisymmetric expansions
authorStieglmeier, Tropea, Weiser, Nitsche
year1989
typeEXP
flow_tagaxisymmetric, separated, varying_cross_section