adb logcat -c

Usage: logcat [options] [filterspecs]
options include:
  -s              Set default filter to silent.
                  Like specifying filterspec '*:S'
  -f <filename>   Log to file. Default is stdout
  -r <kbytes>     Rotate log every kbytes. Requires -f
  -n <count>      Sets max number of rotated logs to <count>, default 4
  -v <format>     Sets the log print format, where <format> is:


                      brief color long printable process raw tag thread
                      threadtime time usec


  -D              print dividers between each log buffer
  -c              clear (flush) the entire log and exit