= Solution
In the <Giesekus model>, $\lambda$ is the <viscoelastic relaxation time>: after deformation stops, polymeric stress relaxes on that timescale. The parameter $\eta$ has dimensions of <dynamic viscosity> and is the model's zero-rate viscosity scale. Since $\lambda\boldsymbol\tau^2/\eta$ already has dimensions of stress, $\alpha$ is dimensionless.
The symbol $\overset{\triangledown}{\boldsymbol\tau}$ is the <upper-convected derivative>
$$
\boxed{
\overset{\triangledown}{\boldsymbol\tau}
=\frac{D\boldsymbol\tau}{Dt}
-(\nabla\mathbf u)\boldsymbol\tau
-\boldsymbol\tau(\nabla\mathbf u)^T}.
$$
The convective and velocity-gradient terms account for translation, rotation, and affine stretching of material elements. They make the constitutive law objective under time-dependent rigid changes of observer.
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