What is viscous dissipation and where is it important? Is it effected by Mach No.? By how much magnitude this effects temperature distribution for supersonic flow?

Viscous dissipation occurs only in turbulence flows.

For example in the turbulent energy equation First order k-e model, it is described as the rate in which turbulent kinetic energy is converted into thermal Kinetic Energy. A more precise definition would be as follows, the Viscous Dissipation term is the destruction of fluctuating velocity gradients by the action of viscous stresses.

Smaller and Smaller Eddies are dissipated by the molecular viscosity near the wall. So it would be very important in the near solid boundaries.

Hope this is in some way helpful.

I would define viscous dissipation as the transformation of kinetic energy to internal energy (heating up the fluid) due to viscosity. This includes both turbulent kinetic energy and mean flow kinetic energy.

Thank you for your responses, but these two responses are brief and also there is no difference b/w dissipation and viscous dissipation. Would anyone please explain these in more detail? Thanks, Robert

Dissipation means the same as viscous dissipation.

Usually when you talk about dissipation in fluid dynamics you mean dissipation of energy, although you can also talk about dissipation of vorticity etc. sometimes.

In a viscous fluid flow the viscosity of the fluid will take energy from the motion of the fluid (kinetic energy) and transform it into internal energy of the fluid. That means heating up the fluid. This process is partially irreversible and is referred to as dissipation, or viscous dissipation.

For a turbulent flow dissipation includes both dissipation of turbulent motions (energy) and mean flow motion. However, the turbulent dissipation is usually completely dominating.

Dissipation is high in regions with large gradients (boundary layers, shear layers etc.) and also in regions with very high turbulence levels (wakes etc.).

Hope that helps.

"Dissipation" on its own is a slightly imprecise term and needs placing in context in order to hone its meaning (even then it can still be a bit wooly). One usually refers to dissipation of something. "Viscous dissipation" usually refers to the work done by the velocity against the viscous stresses - ie an irreversible process where mechanical energy (useful) is turned into thermal engery (not so useful). But some mild confusion can arise concerning mean/fluctuating/instantaneous components in time-averaged equations of turbulent motion and sometimes the term labelled "viscous dissipation" contains a transportive component. At high Mach numbers (in air - the Reynolds number is also a significant parameter) there is a substantial temperature rise in the boundary layer due to "viscous dissipation".

Is there any difference b/w aerodynamic heating and viscous heat dissipation? waiting for reply , Julia

The terms are slightly imprecise when out of context. You would have to state in terms of physical processes what was precisely meant by the two expressions (which would of course answer your own question). It is certainly possible that the terms differ in the context that they were used since the change in state of the fluid can give rise to temperature gradients and heat flow (as can other things). I would suggest looking in a text book or asking the people using the expressions exactly which physical processes they wish to embrace with the expressions. Sorry to duck the question but a definitve answer could be misleading.

How viscous dissipation and kinetic energy of turbulence are related? Also its effect on static temperature and density? Thanks, Daniel

I would suggest looking in a good text book to answer this sort of question.

Generally, viscous dissipation is taken to be the irreversible transfer of mechanical energy to heat by the flow working against the viscous stresses. This occurs at all lengths scales but is dominanted by the contribution from the Kolmogorov scales which are the smallest and most intense.

If one takes a Reynolds averaged view, the flow does work against the Reynolds stresses and the viscous stresses (but because the work done term now involves mean rather fluctuating velocity gradients the second term is now very small at high Reynolds numbers).

The work done against the Reynolds stresses is the transfer of energy from the mean flow into the turbulence. It is the source term in the kinetic energy of turbulence transport equation. The viscous dissipation is the sink term in the transport equation (plus the small direct contribution from the mean flow). The physical linkage between the two is the energy cascade where vortex stretching progressively reduces the length scale and increases the velocity scale of the turbulent motion. However, in a typical RANS turbulence model (eg k-e) both the source and sink terms are linked directly to the work done against the Reynolds stesses.

The energy transferred to heat will raise the temperature of the fluid. The effect on density will depend on Mach No and what the flow is doing.

I suspect I have not adequately answered the question by introducing concepts with which you are unfamiliar. I would, again, suggest looking in a text book and filling in your own gaps.

[marry]

viscous dissipation is also important in the flow of fluids having high viscosities. temperature of the fluid increases because of it.

[sairamum]

Can some one tell me the favorable conditions for viscous dissipation to occur in case of flow past cylinder??

[Jade M]

Viscous dissipation can be important in flows of polymers or oils. Non-dimensionalize the energy equation and calculate the dimensionless parameter in the viscous dissipation term.

[misagh]

viscous dissipation or viscous heating is a phenomenon that usually is neglected in macro-scale geometries. but in micro scale the v.d. effect has significant impress on results.

(e.g. in MACRO tube, by increasing mass flow inlet,the heat transfer coefficient{h} increases.But in MICRO tube increasing mass flow will result in reduction of heat transfer coef.{h} ...that's because of v.d effect)

h=q/(Tw - Tbulk) . in micro tubes v.d effect makes the fluid warmer(Tbulk increases)[consider a constant heat flux on tube...so Tw is independent to fluid] so the tempreture difference reduces and by reduction of DELTA T, h increases.

[fkaveh]

What does the dissipation of PDMS mean? Thank you.

Originally Posted by misagh

viscous dissipation or viscous heating is a phenomenon that usually is neglected in macro-scale geometries. but in micro scale the v.d. effect has significant impress on results.

(e.g. in MACRO tube, by increasing mass flow inlet,the heat transfer coefficient{h} increases.But in MICRO tube increasing mass flow will result in reduction of heat transfer coef.{h} ...that's because of v.d effect)

h=q/(Tw - Tbulk) . in micro tubes v.d effect makes the fluid warmer(Tbulk increases)[consider a constant heat flux on tube...so Tw is independent to fluid] so the tempreture difference reduces and by reduction of DELTA T, h increases.

[Zanje Sachin Bhaskar]

How should i give dissipation term in buoyant jet problem?
I am comparing Fluent results with Experimental results but i am not getting dissipation of heat in near jet exit for fluent result.
Suggest me?

[misagh]

What do you mean by "how should i..."
Applying the v.d effect is as easy as ticking the term of viscous heating in energy equation in the solver.
Refards

[Zanje Sachin Bhaskar]

Please see the attachement

[Zanje Sachin Bhaskar]

Here a i am sure about experimental results but the reduction in temperature for fluent result it is less from maximum of 110 degrees to only 108 degrees.

[Zanje Sachin Bhaskar]

But for experimental which is from 110 degrees to 98 degrees which is realistic at distance of x/d=2.5