Mechanics of Continua and Structures
Recall that x(X,t) gives the position vector of the material particle as a function of time t. The velocity of the material particle can simply be obtained by differentiating the position vector of the particle X w.r.t. time, i.e., by partially differentiating x(X,t) w.r.t. time t and holding X fixed. If we denote the velocity of particle X at time t as V(X,t), then
V(X,t)=∂x(X,t)∂t V(X,t)=˙R(t)X+˙t(t)The above velocity field is called the material velocity field, since it depends on the material position vector (and also time, of course). Recall that X and x are related to each other as
x(X,t)=R(t)X+t(t)We can invert the above equation to write X(x,t)=R−1(t)(x−t(t))X(x,t)=RT(t)(x−t(t))
Thus we can write the velocity field as a function of the spatial position vector as,
v(x,t)=˙R(t)(RT(t)(x−t(t)))+˙c(t)v(x,t)=˙R(t)(RT(t)(x−t(t)))+˙q(t) v(x,t)=˙RRTx−˙RRTt(t)+˙t(t)v(x,t)=Ω(t)x+c(t)Ω(t)=˙RRTc(t)=−˙RRTt(t)+˙t(t)