We carried out a QBO experiment, and recorded the flow oscillation clearly. It was
found that to produce a mean flow, the amplitude of the wave must exceed a certain value,
but we could not determine any condition for the oscillation period.

We developed a new method to visualize the internal gravity wave in the QBO experiment
with moíre. This method can visualize waves in a tank of any shape, since the moíre
associated with the shape of the tank does not appear in a single frame of moíre obtained
by this method. As a result, we were able to confirm the internal gravity wave strongly
distorted by the mean flow.

In addition, by using the flow distribution obtained from the experiment, we calculated
the decrease in wave momentum with viscosity, and the amplitude of the propagating wave.
The results agree fairly well with the image of the internal gravity wave obtained
experimentally with the moíre method. The pattern of the convergence of momentum flux is
almost in agreement with the flow acceleration, except in the neighborhood of the membrane
where accurate flow measurement is difficult. This shows that the internal gravity wave in
the mean flow behaves like the WKB approximation.