Experimental Investigations on Microshock Waves and Contact Surfaces

The present work reports on progress in the research of a microshock wave. Because of the lack of a good understanding of the propagation mechanism of the microshock flow system (shock wave, contact surface, and boundary layer), the current work concentrates on measuring microshock flows with special attention paid to the contact surface. A novel setup involving a glass capillary (with a 200 or $300\mu \mathrm{m}$ hydraulic diameter $D$) and a high-speed magnetic valve is applied to generate a shock wave with a maximum initial Mach number of 1.3. The current work applies a laser differential interferometer to perform noncontact measurements of the microshock flow’s trajectory, velocity, and density. The current work presents microscale measurements of the shock-contact distance $L$ that solves the problem of calculating the scaling factor $\mathrm{Sc}=\mathrm{Re}\times D/(4L)$ (introduced by Brouillette), which is a parameter characterizing the scaling effects of shock waves. The results show that in contrast to macroscopic shock waves, shock waves at the microscale have a different propagation or attenuation mechanism (key issue of this Letter) which cannot be described by the conventional “leaky piston” model. The main attenuation mechanism of microshock flow may be the ever slower moving contact surface, which drives the shock wave. Different from other measurements using pressure transducers, the current setup for density measurements resolves the whole microshock flow system.

Figure 1

A microshock flow in a capillary (simplified sketch). Region 1 is in front of the shock; region 2 is between the shock and the contact surface; region 3 is between the contact surface and the expansion fan (or waves); region 4 is the high pressure driver.

Figure 2

Oscillograph traces (with normalized voltage) of the shock flows in the $200\mu \mathrm{m}$ capillary at the representative propagation distance $x$ (coincides with the distance between the exit of the magnetic valve at position zero and LDI at $x$). The dash-dot line indicates the decreasing shock-induced density jump ${\rho }_2/{\rho }_1$. The photovoltage $U(t)$ scales with flow density $\rho (t)$.

Figure 3

The flow trajectories in the 200 and $300\mu \mathrm{m}$ diameter capillaries (from one beam out). Error bars derived from standard deviation are smaller than the symbols here. The theoretical model is from Roshko [15].

Figure 4

Shock-contact distance $L$ as a function of shock location $x_s$. $L$ is derived from the shock wave and contact surface trajectories in Fig. 3 via the relation $L(x_s)=x_s(t)-x_{c,\mathrm{fit}}(t)$.

Figure 5

Shock wave velocity $u_s$ (direct measurement, two beams in) and postshock particle velocity $u_p$ (indirect measurement) as functions of the propagation distance $x$.