Hydrodynamics

Understanding Hydrodynamics

Hydrodynamics is based on the conservation laws of mass, momentum and energy. At high pressure and stress levels, even materials such as metals change volume and shape readily and flow in a manner similar to a fluid.

Prediction of the dynamic behaviour of materials as they flow under the influence of high pressure and stress is of considerable importance in the understanding of weapons. Simulations of the hydrodynamic behaviour of experiments are carried out on AWE’s supercomputers using calculational tools known as hydrocodes. The results from these simulations are verified with experimental measurements.

The regimes of interest are shock wave conditions, ranging in pressure from a few tens of kilobars (Kb) up to several megabars (Mb). (1 Kb is roughly the pressure at the bottom of a mid-ocean trench. The pressure at the centre of the earth is around 3 Mb).

Physics Issue	Example Diagnostics
The initiation and behaviour of high explosives	Ionisation probes High speed cameras Optical probes (under development)
The equation of state of a range of relevant materials	Gas gun, stress gauges, Doppler interferometry
The strength of materials	Taylor Test impact experiments Gas gun
Material ejected from shocked surfaces	Piezo electric probes or Asay foil probes for momentum Heterodyne interferometry (velocity) Optical methods under development in collaboration with Cambridge University (particle size distribution)
The mixing of fluids	Linear and convergent shock tubes
Dynamic friction	Radiography
Material temperature	Optical pyrometry (under development)
Material phase	Quasi-static experiments with Diamond Anvil Cells Emissivity measurement for shock melting (under development)

An example of two of these areas, the mixing of fluids and gas gun experiments, are described in more detail below...

The Mixing of Fluids

When two fluids of differing densities in contact with each other are accelerated, the boundary becomes unstable. Under constant acceleration, the instability is termed Raleigh-Taylor instability.

However, under shock conditions, i.e. an impulsive acceleration, the boundary experiences Richtmyer-Meshkov instability growth. The instability growth leads to turbulent mixing of the fluids across the interface.

This phenomenon can be investigated using a shock tube. A shock tube conventionally takes the form of a strong smooth wall steel pipe, divided into two compartments separated by a diaphragm of thin material.

The gas in the shorter end is pressurised and the device is operated by bursting the diaphragm. The rapidly released compressed gas flows along the pipe and develops into a shock wave.

An example of the data obtained is compared with theoretical calculations (Figure 1).

Figure 1.

The AWE Single Stage Light Gas Gun

The AWE single stage gas gun (Figure 2) uses helium reservoirs pressurised to 260bar to drive a 150g sabot, carrying a flyer plate, at up to 1km/s down a 3 metre long large diameter (70mm) evacuated barrel.

The flyer plate is precisely impacted onto a target plate of the sample material so that 1-D shock loading can be obtained in the sample. Diagnostics are included to allow fundamental shock properties of the sample material to be measured accurately.

The useful data from the experiment is taken in the first few microseconds (millionths of a second) after impact.

Figure 2.