Dear All
Why we have to consider the fluid as moving and the designed system will be the as a stationary?
I mean in realty the system is moving while the fluid is stationary "at most cases"..
Can anybody clarify that to me please?
can you clarify the question? As "stationary" do you mean like a Tank sitting on the ground filled with water? or a truck filled with water moving down the road? or are you referring to piping fluid where the fluid is moving through a stationary installation? or in the case of "the system is moving while the fluid is stationary" are you talking about a axil spinning around a non moving Fluid bath?
I have a feeling that what ever the case is, it's because fluid has a chance to store or change energy internally, and you have to consider how that internal change will affect the system. Solids don't generally store explosive pressures, or change center of gravities while sloshing around, or create water hammer or back pressures or exert forces when turning corners.
Consider I have a swimming robot, which is for sure has a velocity, when I do the flow simulation calculations, the water is the moving part, where as the robot will be fixed with no motions "stationary", the speed of the water is the speed of the robot.
Why we do not let the robot moves as it is and the water is set as stationary?
It is not which, fluid or solid body, is stationery or moving. It is the relative motion between them is to consider because due to this relative motion momentum transfer occurs. This momentum transfer is associated with force (Newton’s third law of motion in collision case). In this relative motion solid body cannot deform but fluid can.
In case of pump/turbine, the impeller is the moment transfer point where moving fluid can impart momentum to impeller acting as turbine or moving impeller can impart momentum to fluid acting as a pump.
In case of vehicle, it is the impeller’s relative motion in the fluid (compressible or incompressible) the vehicle gets the force to propel through the fluid due to momentum transfer.
In simulation of fluid system, FEA is used which is a numerical method to determine the fluid trajectory in reference to solid body and force created in solid surfaces. It is immaterial whether, with respect to some external reference, the solid body or fluid in motion.
It is convenient to use fluid in motion and solid body as stationery to analyze the behavior where the propelling force is known and again this propelling force is created from the relative motion of some part of that solid body with respect to fluid.
You robot in the swimming pool, generates its propelling force due to motion of its hand that is determined first using one FEA. This propelling force will generate motion of robot through water and will come under drag. This is determined in another FEA considering robot as stationery and water as moving. You cannot do both in one go.
I think this is to get a steady velocity of the fluid as the Navier Stoke's equation is very handy in CFD, and it is based on velocity at a point in space and time. So, the only way or probably a better way is to assume the the body constant and the fluid flowing with a constant velocity.
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