Non-dimensionalization and scaling of the Navier–Stokes equations
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In fluid mechanics, non-dimensionalization of Navier–Stokes equation is the conversion of the Navier–Stokes equation to a form which is easier to use and reducing the number of free parameters in the problem to be studied. The non-dimensionalized Navier–Stokes equation is beneficial to use when posed with similar physical situations that is problems where the only change are in the basic dimensions of the system. Scaling of Navier–Stokes equation refers to the process of selecting the proper scales for non-dimensionalization of the equation. Since the final equation needs to be dimensionless, using the parameters and constants of the equation a suitable combination of these is found having the same dimensions as that of the variables in the equation. As a result of this combination the number of parameters to be analyzed is reduced and the results are obtained in terms of scaled variables.
Need for non-dimensionalization and scaling
In addition to reducing the number of parameters, non-dimensionalized equation helps to gain a greater insight into the relative size of various terms present in the equation. Following appropriate selecting of scales for the non-dimensionalization process, this leads to identification of small terms in the equation. Neglecting the smaller terms against the bigger ones allows for the simplification of the situation. For the case of flow without heat transfer, the non-dimensionalized Navier–Stokes equation depend only on the Reynolds Number and hence all physical realizations of the related experiment will have the same value of non-dimensionalized variables for the same Reynolds Number.
Scaling helps provide better understanding of the physical situation, with the variation in dimensions of the parameters involved in the equation. This allows for experiments to be conducted on smaller scale prototypes provided that any physical effects which are not included in the non-dimensionalized equation are unimportant.
The Navier–Stokes equation is written as:
The above equation can be dimensionalized through selection of appropriate scales as follows:
Scale dimensionless variable Length L Velocity U Time L/U Pressure: there is no natural selection for the pressure scale. Where dynamic effects are dominant i.e. high velocity flows
Where viscous effects are dominant i.e. creeping flows
Substituting the scales the non-dimensionalized equation obtained is:
This is only valid if there is a free surface. For flows where viscous forces are dominant i.e. slow flows with large viscosity, a viscous pressure scale μU/L is used. In the absence of a free surface, the equation obtained is
Density variation due to both concentration and temperature is an important field of study in Double diffusive convection"If density changes due to both temperature and salinity is taken into account then some more terms add to the Z-Component of momentum as follows: 
Where S is the salinity of the fluid, βT is the thermal expansion coefficient at constant pressure and βS is the coefficient of saline expansion at constant pressure and temperature. Non dimensionalizing using the scale:
where ST, TT denote the salinity and temperature at top layer, SB, TB denote the salinity and temperature at bottom layer, Ra is the Rayleigh Number, and Pr is the Prandtl Number. The sign of RaS and RaT will change depending on whether it stabilizes or destabilizes the system.
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This book contains several examples of different non-dimensionalizations and scalings of the Navier–Stokes equations
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