Wakes and vortex streets generated by translating force and force doublet: laboratory experiments

Wakes and vortex streets such as those occurring behind towed or self-propelled bodies are generated by moving localized forces in a viscous fluid at moderate values of the Reynolds number, $\hbox{\it Re}\,{\sim}\,10^{2}$. The forcing is provided by an electromagnetic method and allows us to create a ‘virtual’ body without introducing any solid objects into the fluid. Characteristics of stable and unstable wakes, in particular the shedding frequency, are measured in the space of control parameters, namely the magnitude of the forcing and the speed of translational motion of the forcing. The results for a single force presented in the dimensionless form of the Strouhal number demonstrate quantitative similarity to those for the classical flow around a cylinder. The problem considered here has an extra degree of freedom compared to the problem of the flow around a cylinder and exhibit a wider array of different regimes. These regimes are documented in both our visualization experiments and particle image velocimetry measurements.