Figure 12. Pressure gradient magnitude contours. ( a ) 13 μs; ( b ) 22 μs; ( c ) 121 μs; ( d ) 504 μs. Regions surrounded by black Figure 12. Pressure gradient magnitude contours. ( a ) 13 µ s; ( b ) 22 µ s; ( c ) 121 µ s; ( d ) 504 µ s. Regions surrounded by black lines represent fresh gas regions. lines represent fresh gas