liquid sloshing is primarly modelled for two cases.One in which tank is provided some reciprocatng motion, this motion you can easily formlate in udf using sinusoidal wave like Asinwt where w is sloshing natural frequency,

other way is to have a sudden braking of tank in motion . this produces a impact hit on front and back side of wall. this you can give using a table where for a certain span of time acceleration becomes negative, then for a very short span it becomes positive followed by a zero. you can import this table in udf.

For second case you can follow this as your acceleration profile

Thanks for this help..for udf can we use any language?