acoustic shadow over ewell?

In early July 1863, the Union and Confederate forces faced off at the town of Gettysburg, Pennsylvania. On the second day of the battle on July 2nd, the commanding general of the Army of Northern Virginia, General Robert E. Lee had a simple plan to dislodge the Union forces entrenched in front of him on the Cemetery Ridge. The First Army Corps under the command of Lieutenant General James Longstreet would attack the Union forces on the left flank, while the Second Army Corps under the command of Lieutenant General Richard S. Ewell would demonstrate on the right flank as a diversion to keep the enemy guessing. The signal for Ewell to initiate his diversion was the sound of Longstreet's artillery barrage. It took Longstreet all morning to assemble two divisions (McLaws and Hood) for the attack. The offensive was launched in the afternoon around 4pm, but General Ewell didn't hear Longstreet's cannons and therefore, did not commence his diversionary attack. This gave the Union commander, Major General George G. Meade clear indication on where the actual attack was coming from and he was able to move troops on his left flank to defend against Longstreet's offensive (reserves from the 5th Army Corps under the command of Major General George Sykes and elements from Major General John Sedgwick's 6th Army Corps were rushed to join the fight as they arrived on the battlefield). According to Ross (1999), the atmosphere was probably upwards refracting (due to surface heating at the ground), which, created an acoustic shadow over Ewell. In the end, Meade was able to repulse Longstreet's attack - some say due to the inaction of Ewell, that afternoon. An analysis of Ross's hypothesis is reported in Ahmad et al. (2008).

longstreet's cannons (july 2nd, 1863)



Locations of the batteries under Alexander, Cabell and Henry and Ewell's HQ are shown in the left panel. The unstructured adaptive grid is shown in the right panel. The mesh has approximately 25000 cells with edge lengths varying from 10m to 100m.

details

Ahmad, N., D. Bacon, T. Dunn, M. Hall, A. Sarma, T. Wait, 2008: Acoustic Ray-Tracing on Unstructured Adaptive Grids. American Institute of Aeronautics and Astronautics. AIAA Paper 2008-62.

Ross, C., 1999: Outdoor Sound Propagation in the U.S. Civil War, Echoes, Vol. 9.