Dear Oualid, solid materials are viscoelastic, this means that they feature intermediate characteristics between purely elastic and purely viscous behaviour:

"Viscoelasticity is a property of materials that exhibit time-dependent strain [41]. Perfectly viscous materials exhibit stress proportional to strain rate, while perfectly elastic materials feature stress proportional to strain, the proportionality constant being the elastic (or Young’s) modulus. Viscoelastic materials feature intermediate characteristics between purely elastic and purely viscous behaviour, i.e., they show both behaviours when undergoing deformation.

In crystalline solids, elasticity usually involves the stretching of atomic bonds (i.e., atomic displacements) along specific crystallographic planes, whereas viscosity and viscoelasticity involve relaxations associated with diffusion. For example, this mechanism enables relaxation phenomena consisting in atomic rearrangements, large-scale cooperative motion of atomic groups, or localized atomic motion without collective atomic rearrangements or changes in chemical order [42], e.g., movements of the network itself or diffusion of a mobile species [43, 44]. Materials may exhibit viscoelastic relaxations in response to mechanical, electrical or temperature perturbations, and the relaxation processes are manifested by a transient response of physical or thermodynamic properties (e.g., enthalpy, volume, strain or stress). In any case, relaxations result in frictional energy loss appearing as heat [45].

Whether the behaviour of a material is closer to purely elastic or purely viscous depends mainly on temperature and the excitation frequency (strain rate), but it may also depend significantly on test and environmental conditions such as the pre-load, dynamic load, environmental humidity, etc. [2]. To characterize the response of a material, samples are usually excited dynamically under controlled temperature, frequency and amplitude of the loading, and the strain-stress behaviour is recorded. For instance, they are often strained or stressed sinusoidally with a Dynamic-Mechanical Analyser (DMA). For an ideal elastic material, stress and strain are in phase, and the energy lost as heat per cycle is null. For an ideal viscous material, stress and strain are 90º out of phase. In reality, viscoelastic behaviour is more common. Thus, for most of the materials there is usually an intermediate phase lag between stress and strain (i.e., they are out of phase) and a certain amount of frictional energy loss (see Fig. 3). Polymers, polymer solutions and dispersions, metals and amorphous materials (organic and inorganic) are examples of materials that exhibit viscoelastic behaviour.

The dynamic behaviour of linear viscoelastic materials can be described by the complex modulus approach [2]. In particular, the parameters commonly utilized for the characterization of the viscoelastic properties are the two components of the complex (or dynamic) tensile modulus, and the loss tangent. The elastic (real) component of the tensile modulus is the storage modulus E’, which is a measure of the deformation energy stored by the material. The viscous (imaginary or plastic) component of the tensile modulus is the loss modulus E”, which accounts for the energy dissipation due to internal friction, i.e., the frictional energy loss, during relaxation processes [1]. Finally, the loss tangent is the ratio energy loss-to-energy stored, i.e., the ratio loss modulus-to-storage modulus (see Fig. 4)."

Hope this is useful,

Jose

References:

[1] Nowick, A.S., and Berry, B.S., "Anelastic relaxation in crystalline solids," Academic Press, New York; London, 1972, pp. 677.

[2] Rao, M.D., "Recent applications of viscoelastic damping for noise control in automobiles and commercial airplanes," Journal of Sound and Vibration, Vol. 262, No. 3, 2003, pp. 457-474.

[41] Meyers, M.A., and Chawla, K.K., "Mechanical Behavior of Materials," Cambridge University Press, Cambridge, UK, 2009,

[42] Spaepen, F., "Physics of defects," Balian, R. Kleman, M. and Poirier, J.P. eds., North-Holland Pub. Co., Amsterdam, The Netherlands, 1981, pp. 857.

[43] Hopkins, I.L., and Kurkjian, C.R., "Physical acoustics: principles and methods," Mason, W.P. ed., Academic Press, New York, NY, USA, 1965,

[44] Angell, C.A., "Dynamic Processes in Ionic Glasses," Chemical Reviews, Vol. 90, No. 3, 1990, pp. 523-542.

[45] Suh, D.W., and Dauskardt, R.H., "Mechanical relaxation time scales in a Zr-Ti-Ni-Cu-Be bulk metallic glass," Journal of Materials Research, Vol. 17, No. 6, 2002, pp. 1254-1257

All media have viscosity. Bluntly put, given enough duration everything flows, even the mountains! Liquid forms of the metals have well defined viscosity. May be use the relation between dynamic viscosity and kinetic viscosity to extract the former.

Dear Oualid, solid materials are viscoelastic, this means that they feature intermediate characteristics between purely elastic and purely viscous behaviour:

"Viscoelasticity is a property of materials that exhibit time-dependent strain [41]. Perfectly viscous materials exhibit stress proportional to strain rate, while perfectly elastic materials feature stress proportional to strain, the proportionality constant being the elastic (or Young’s) modulus. Viscoelastic materials feature intermediate characteristics between purely elastic and purely viscous behaviour, i.e., they show both behaviours when undergoing deformation.

In crystalline solids, elasticity usually involves the stretching of atomic bonds (i.e., atomic displacements) along specific crystallographic planes, whereas viscosity and viscoelasticity involve relaxations associated with diffusion. For example, this mechanism enables relaxation phenomena consisting in atomic rearrangements, large-scale cooperative motion of atomic groups, or localized atomic motion without collective atomic rearrangements or changes in chemical order [42], e.g., movements of the network itself or diffusion of a mobile species [43, 44]. Materials may exhibit viscoelastic relaxations in response to mechanical, electrical or temperature perturbations, and the relaxation processes are manifested by a transient response of physical or thermodynamic properties (e.g., enthalpy, volume, strain or stress). In any case, relaxations result in frictional energy loss appearing as heat [45].

Whether the behaviour of a material is closer to purely elastic or purely viscous depends mainly on temperature and the excitation frequency (strain rate), but it may also depend significantly on test and environmental conditions such as the pre-load, dynamic load, environmental humidity, etc. [2]. To characterize the response of a material, samples are usually excited dynamically under controlled temperature, frequency and amplitude of the loading, and the strain-stress behaviour is recorded. For instance, they are often strained or stressed sinusoidally with a Dynamic-Mechanical Analyser (DMA). For an ideal elastic material, stress and strain are in phase, and the energy lost as heat per cycle is null. For an ideal viscous material, stress and strain are 90º out of phase. In reality, viscoelastic behaviour is more common. Thus, for most of the materials there is usually an intermediate phase lag between stress and strain (i.e., they are out of phase) and a certain amount of frictional energy loss (see Fig. 3). Polymers, polymer solutions and dispersions, metals and amorphous materials (organic and inorganic) are examples of materials that exhibit viscoelastic behaviour.

The dynamic behaviour of linear viscoelastic materials can be described by the complex modulus approach [2]. In particular, the parameters commonly utilized for the characterization of the viscoelastic properties are the two components of the complex (or dynamic) tensile modulus, and the loss tangent. The elastic (real) component of the tensile modulus is the storage modulus E’, which is a measure of the deformation energy stored by the material. The viscous (imaginary or plastic) component of the tensile modulus is the loss modulus E”, which accounts for the energy dissipation due to internal friction, i.e., the frictional energy loss, during relaxation processes [1]. Finally, the loss tangent is the ratio energy loss-to-energy stored, i.e., the ratio loss modulus-to-storage modulus (see Fig. 4)."

Hope this is useful,

Jose

References:

[1] Nowick, A.S., and Berry, B.S., "Anelastic relaxation in crystalline solids," Academic Press, New York; London, 1972, pp. 677.

[2] Rao, M.D., "Recent applications of viscoelastic damping for noise control in automobiles and commercial airplanes," Journal of Sound and Vibration, Vol. 262, No. 3, 2003, pp. 457-474.

[41] Meyers, M.A., and Chawla, K.K., "Mechanical Behavior of Materials," Cambridge University Press, Cambridge, UK, 2009,

[42] Spaepen, F., "Physics of defects," Balian, R. Kleman, M. and Poirier, J.P. eds., North-Holland Pub. Co., Amsterdam, The Netherlands, 1981, pp. 857.

[43] Hopkins, I.L., and Kurkjian, C.R., "Physical acoustics: principles and methods," Mason, W.P. ed., Academic Press, New York, NY, USA, 1965,

[44] Angell, C.A., "Dynamic Processes in Ionic Glasses," Chemical Reviews, Vol. 90, No. 3, 1990, pp. 523-542.

[45] Suh, D.W., and Dauskardt, R.H., "Mechanical relaxation time scales in a Zr-Ti-Ni-Cu-Be bulk metallic glass," Journal of Materials Research, Vol. 17, No. 6, 2002, pp. 1254-1257

Dear Oualid,

    Viscosity is a feature for fluids or solid melts. You can find helpful answer for your problem in COMSOL Software by Jeff Hiller at the following link.

Best regards

Khaled

https://www.comsol.com/community/forums/general/thread/73111/

Thank you so much Jose for your explanation and the list of References that you recommanded to me I really appreciated your assistance ^^

There now I know that there exists a viscosity concerning solid materials which is called " Viscoelasticity " ...

And then there is another important point as I read in your passage ...

You approached on mechanical, electrical and temperature disturbances, in my case it is a metal plate supplied with continuous high voltage but I don't think that the latter can create any deformation in the plate on macroscopic scale but I ignore if it influences in microscopic scale ?

Thanks Khaled for your help me too I was convinced about this information but I found some articles giving its value for steel, aluminium or even copper in it solid state that what made me confused ^^

I believe your solid materials are "mathematically" solid. COMSOL does not know it is solid unless you tell it so. Simply specify it as solid, say, teat the solid-fluid interface as boundary of the computational domain, and totally ignore the your solid materials.

Thank you very much Mr Shiuh for your time and consideration.

I have followed your suggestion but in COMSOL Multiphysics for each type of physics you add it asks you to put the initial values that are needed in this study. For example if we choose Laminar flow, it asks you to enter the Dynamic viscosity for each domain you create ^^

I have searched more in my model and when you affect each material for it's adequat domain there you can specify in material properties if it is " Solid " or "Non-Solid " and it's still the same situation as always, asking for the dynamic viscosity ^^ ...

Which version of COMSOL? Can you upload your file?

I am using 5.2. Here I upload an example fyi

I use the version 5.0, and here is my model ^^

In Laminar Flow (spf)-> Setting -> Domain Selection -> Only select domain 2. 

This should solve your problem.

See attached figure

Qualid,

Why do you set up those two thin walls? If you are only interested in the fluid flow, you don't need it. You can only setup your geometry for the fluid domain and specify the upper and lower boundaries as "wall".

Hi Mr. Shiuh and thanks again for cheking out at my model but I used this thin walls because they represent the plates and my next step, I will apply a voltage in this plates that's why I added them ...

If you have another suggestion, I'm listening ^^

Best regards Oualid,

Then you need to have a multiphysics. In the fluid module, take away the two walls. In the DC/AC (?) module, only include the two walls.

Please make sure you have the right physics selected for each geometry area, comsol will ask you about the dynamic viscosity of copper for example if you have fluid flow physics selected in the area where copper material is set.