in the test filter was introduced briefly, then the outflow velocity profile measured at the outlet of the fan duct used in the
experiment was presented. Following to these basic experimental information, unexpected vibrations occurring in the two
types of filter – filter I of length L¼1.8 m and filter II of L¼3.7 m – and a measure to suppress the unexpected vibration for
each test filter was presented, respectively. The results obtained in this study can be summarized as follows:
I. Characteristics of outflow from the duct outlet.
Outflow from the duct is not parallel to the duct axis but is inclined by about 251 from the left to the right
direction toward the duct outlet plane, i.e., the outflow is vortical. In addition, it is not uniform in the radial
direction but is almost zero at the center of the duct and has a single peak in the radial direction.
The outflow from the duct outlet contained velocity variation components corresponding to the number of revolutions
of the fan (¼f0¼26 Hz) and four times this value, i.e., 4f0(¼104 Hz); four is the number of blades (N¼4).
The outflow velocity dropped markedly after exiting the duct outlet.
II. Effects of roll core panel.
With the roll core panel at the duct outlet, vortical outflow was rectified to flow parallel to the duct axis.
Although the core panel with small cell size was effective for rectification of the outflow, the velocity of the
rectified flow was reduced relative to that with the core panel with larger cell size.
Although the outflow velocity without the core panel rapidly dropped in the neighborhood of the duct outlet, the
outflow velocity maintained its magnitude for a longer distance l0 with use of the core panel. The effect of the
number of core panels was insignificant except in the outlet region, l0E0–80 cm.
Fig. 25. Transition of filter cross-section shape with interior shielding area.
Fig. 26. Effect of interior shielding on filter shape. (a) Without interior shielding; (b) interior shielding on the top area (c) wider interior shielding.
M. Chiba et al. / Journal of Fluids and Structures 27 (2011) 1392–1410 1409
III. Vibration characteristics of the filter I (L¼1.8 m) and vibration suppression method.
The vibration amplitude showed the maximum value at l0E0.3 m, i.e., in the neighborhood of the duct outlet, in
the longitudinal direction, when the roll core panel was not used.
The predominant component of the vibration of the filter was seen at 8.0 Hz, with other components at 12.8 Hz
(E(1/2)f0) and 25.5 Hz (¼f0).
Vibration of the filter was suppressed with the roll core panel at the duct outlet and rectifying the outflow.
Therefore, unexpected vibration was due to marked collision of vortical outflow against the thin soft filter at the
duct outlet.
With the roll core panel, the predominant frequency in the filter vibration decreased slightly to 7.2 Hz for S-140
and to 6.3 Hz for S-85.
The predominant circumferential vibration mode was N¼2 without the core panel and N¼1 with the S-85
core panel.
IV. Vibration characteristics of the filter II (L¼3.7 m) and vibration suppression method.
As filter II was about double the length of filter I, the surface of the filter did not fully expand along its whole
length due to lack of the power of the fan used in the experiment.
In contrast to filter I, vibration could not be suppressed by using the roll core panel and rectifying the duct flow,
and the amplitude of vibration increased and the peak position moved to the tip end of the filter in the
longitudinal direction.
Vibration in filter II represented ‘‘fluttering,’’ in which the deformed part of the filter fluttered due to internal flow.
Therefore, by shielding the outflow from the surface of the tip part of the filter with a thin membrane, we could
suppress the vibration along the whole length of the filter.
These obtained results represent useful experimental data to clarify this phenomenon from the theoretical viewpoint in
future studies.