 there are two main types of how your flow in a pipe or around some other structure can go. Option one is your flow is laminar meaning your flow particles kind of travel mostly parallel and the ones there at let's say at the bottom of the pipe will not suddenly go up to the top of your pipe the other option is if there's a lot of turbulence and then your flow paths are chaotic a lot of little eddies and your flow does kind of all type of different things so option one is laminar and option two is turbulent now depending on the application you might want laminar you might want turbulent if you want your fluids to mix or for example in a heat exchanger you want the heat to be easily transferred to the outside of the pipe then you maybe want turbulent if however you want to have easy calculations or not have turbulence as in the case of an airplane wing then you want laminar so how do you know ahead of time which type of flow you're going to get well we have a good indicator for this one which is the so-called Reynolds number now the Reynolds number can be calculated as the density of your fluid times the speed at which your fluid is traveling times a typical distance now in the in the case of a pipe that will be your diameter and then you divide it by the viscosity which is another material constant that you can look up for your fluid now if your Reynolds number ends up being smaller than 2000 most likely you're going to have laminar flow if however your Reynolds number is much bigger say three thousand some sources say four thousand if you can see some saying ten thousand so the large-year Reynolds number the more chance you have to have turbulent flow if you're in between so let's say Reynolds number of two and a half thousand you will have an unstable flow which can switch from one to the other which is probably the least thing that you want in any application you don't want your laminar flow to suddenly become turbulent or the other way around either one you would prefer laminar or you would prefer turbulent