The ability to model the interaction between fluid and structural physics in ANSYS Multiphysics allows user to model mircofluidic pumps, fluid flow in elastic channels and also understand how a fluid damps mechanical motion at this scale. This physics coupling is commonly referred to as Fluid Structure Interaction (FSI). In ANSYS Multiphysics, FSI scenarios are those that involve the coupling of fluid mechanics and structural/ thermal/ coupled field mechanics. The deformations/temperature of a given solid are computed simultaneously with the flow and heat-transfer variables of a fluid that surrounds the solid.
The key features of ANSYS Multiphysics FSI are:
The elements used in the fluidic domain use an Alternate Langrangian Eulerian (ALE) formulation that allows them to undergo extreme deformation (aspect ratios of the order of 10,000:1). The provides for an extremely robust mesh morphing algorithm to handle the moving /deforming fluid domain. The user specifies the displacement & velocity time history for moving body and turns ALE on. The mesh morphing is elasticity based. Excellent agreement with benchmark / experimental results have been shown. Both lift and drag forces can be computed, and both incompressible and incompressible with mild compressibility effects (see below) are handled. Equivalent reduced order resistance and damping terms can be extracted.
Here are some ANSYS FSI application examples:
This animation show a pressure pulse in a fluid raveling down an elastic artery. The fluid could be Newtonian or non Newtonian (e.g. blood):
The following example is a two dimensional axisymmetric model of a piezoelectric micropump (The true pump is a circular disc diaphragm pump). The axis of symmetry is the Y axis, left hand side of this image. The piezoelectric material is a thin strip along the top oedge of pumps diaphragm. The inlet/outlet of the pump is at the narrow part of the taper on the right hand side of this image.
Animation of the pump operating, fluid velocity vectors are plotted.
This final images shows graphically how the pump performs with one "pump impulse" applied. It shows how the motion is damped over time when run at atmospheric pressure and in a vacuum.
End view of a three dimensional MEMS parallel plate capacitor / mirror assembly. The upper mirror plate rotates about an axis normal to viewing angle. The image shows the pressure contours computed when a harmonic displacement motion is imposed on the upper plate.
ANSYS FLOTRAN’s Arbitrary-Lagrangian-Eulerian (ALE) formulation, which is sometimes used to simulate squeeze-film damping fluid-structure interaction (FSI) applications, can treat mildly compressible flows of liquids and gases. For these cases, the user must input a suitable value for bulk modulus parameter, denoted by the symbol b. This bulk modulus parameter is used in the transient algorithm for incompressible flows, yet it accounts for mild compressibility effects, i.e., non-infinite speeds of sound.
The speed of sound, or the acoustic speed, is the speed at which a sound wave or small pressure disturbance propagates in a fluid medium. For any fluid continuum, the acoustic speed, denoted by a, is given by
where ρ is the fluid density, and Ks is the isentropic bulk modulus, which relates the change in pressure within the fluid to fractional changes in density, or
Combining the above relationships yields
Here, β is FLOTRAN’s bulk modulus parameter, input with FLDATA16, BULK, BETA, Value.
If the fluid is a liquid, β should be input as the liquid’s isentropic bulk modulus (available in fluids handbooks) divided by its density. From the above expression, it is apparent that β should have dimensions of velocity squared (L2/ T2).
If the fluid is an Ideal gas, undergoing an isothermal compression/expansion, it can be shown that
Thus,
where R is the Ideal Gas Constant, and T is the absolute temperature. Once again, β should have dimensions of velocity squared.