Figure 11. Pressure gradient magnitude contours. ( a ) 296 μs; ( b ) 303 μs; ( c ) 309 μs; ( d ) 321 μs. Regions surrounded by black Figure 11. Pressure gradient magnitude contours. ( a ) 296 µ s; ( b ) 303 µ s; ( c ) 309 µ s; ( d ) 321 µ s. Regions surrounded by black lines represent fresh gas regions. lines represent fresh gas