If not already done, wash and dice rhubarb, and set aside
Place water in a large pot and bring to a boil
Add brown sugar and 1 cup granulated sugar. Add rhubarb, and remove from heat
Allow to cool to lukewarm (110-115 degrees Farenheit)
Add yeast, stir. Transfer mixture to a large non-metallic container such as a glass jar
Cover lightly, and leave to sit overnight at room temperature (or until little bubbles appear around the edges near the surface)
Sterilize bottles, allow to cool. Place 1 tsp granulated sugar (or more, to taste) in each bottle.
Place a few raisins in each bottle.
Strain sima into bottles and cover or cork tightly.
Leave to sit at room temperature until all raisins have risen to the top of the sima. This could take anywhere from 8-24+ hours depending on the temperature of your kitchen
Transfer to the fridge to stop fermentation and chill. Enjoy!