英语翻译It is assumed in the theory of ultrasonic flowmeters tha

英语翻译
It is assumed in the theory of ultrasonic flowmeters that a planar wave is radiated [i] ;
then the forward propagation (in the flow direction) and the backward propagation (against
the flow) are symmetrical,and therefore there is complete balancing of the diffraction
effects taken over the differential time,which is equal to the difference between the times
of propagation from the source to detector and back again (when the source acts as a detector
and vice versa).
In fact,the wave in the near zone (the Fresnel zone) is not exactly planar,and it goes
gradually over to spherical form as the distance from the source plate increases [2].However,
the symmetry of the forward and backward waves is then violated,and one therefore expects
that a diffraction effect will be present.We show below that this idealization of the
wave pattern can result in considerable error.
We omit the detailed formulation of the equations and the solution and merely note that
the vibrations of the source plate are represented,as usual,as those of a piston; we also
neglect the effects of any pockets in the walls,reflections from the boundaries of the moving
and immobile media,acoustic-wind effects,and the like.The following are the expressions
for the mean pressure set up at the detection plate and for the time of the signal to
progagate from the radiator to the detector:(这里是1个公式,省略）
where Re is the real part of the expression appearing within the braces,0 is the density of
the medium,vo is the amplitude of the vibrational velocity,c is the speed of sound in the
medium,d2 = I + (r/z)cos 8,z is the distance between the emitting and receiving plates
(these are coaxial and parallel),B is the angle between the axis of the acoustic channel and
the axis of the tube,r = vro/c,v is the flow speed of the fluid,（公式） d is the
diameter of the pipe,m is the circular frequency,k is the wave number,（公式*n）
where a is the radius of the plate (this is taken as the same for the emitting and receiving
plates for simplicity,but there is no difficulty in discussing plates differ~mg in radii),
（公式）,is the vibrational frequency of the emitting plate,and（公式）
The triple sums in the expressions for and N2 may be neglected if the flow speed is
low,since the contributions for these become negligible.

按照超声流量计的理论,假定平面波是放射状的,那么正向传播（流体方向）与反向传播（逆流方向）是对称的,因此在某一时段内有完全平衡的衍射效果.该时段等于从源头传播到探头并反传回来（当源头当做探头时,反之亦然）.
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哎 哥不能晚上加班给你做翻译
俺是专业翻译人员,觉得也不简单
先睡觉了……