Location: Covilhã, Portugal (and sometimes in Évora)
Post
....just remebered in certain cases (under mach 0,1) liquids are also treated as uncompressed, because the error is below 5%....but for a correct approach to a problem you should always consider them compressible.
Although both aero and hydro CFD tend to be based on Navier-Stokes descitization, I've heard that hydrodynamics specific codes have evolved differently due to the more laminar nature, as well as different needs (i.e. wake, bow/water interface, and specifics). So they tend to have models based on fits to experiemental data included.
Although CFD mostly started in aero, the rapid development into a large range of fluid viscosity models, turbulence treatements (applicable reynolds numbers regimes), they are getting quality enough results to be used in some hydro applications as well.
Location: Covilhã, Portugal (and sometimes in Évora)
Post
Well....I was going to post that my fluids teacher said that building and hydrodynamic object might not be the same as buiding an aerodynamic object (sometinh like this).....but wasn't sure about it...and didn't know how to explain it.....but the previous post mentions something close to it.
Conventionally under Mach = 0.3 the compressibility effects can be neglected (although obviously it depends by the application). Without entering in the details you can understand it knowing that when you take in account compressibility in the relationships there’s a coefficient : (1+ 0.5 (gamma – 1) M^2), gamma is the ratio of specific heats, 1.4 for air => if the coefficient is close to 1 the difference with an incompressible case is limited, if M = 0.3 the coefficient is 1.018.
As an evaluation, if the air temp is 15°C, M=0.3 is about 100 m/s => 360 km/h.
Generally the speed for turbulent flow to occur in water is about 15 times the speed for turbulent flow in air. So its a lot harder to have laminar air flow, versus laminar water flow.
Also air becomes compressible at about 100m/s (360 kph) but water flow is compressible at about 450 m/s (1620 kph) which is pretty difficult to obtain.
sirus wrote:
Generally the speed for turbulent flow to occur in water is about 15 times the speed for turbulent flow in air.
The ratio depends a lot by the temperature of both fluids, exactly because viscosity (and also cinematic viscosity mu/rho that is the factor dependant by the fluid in the Reynolds number) has a opposite trend vs. temp in a liquid and in a gas. At 15°C (both fluids) the ratio is 12.71, 15 @ 20°C and, for example, 39 @ 60°. If water is just a bit above 0°C and air is at –30°C, the ratio is only about 6.
