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9.2.2: Basic Units Blocks

In Thermo–Fluid science there are several basic physical quantities which summarized in Table 9.1. In the table contains two additional physical/basic units that appear in magnetohydrodynamics (not commonly use in fluid mechanics). Many (almost all) of the engineering dimensions used in fluid mechanics can be defined in terms of the four basic physical dimensions MM, LL ,tt and θθ. The actual basic units used can be S.I. such as kilograms, meters, seconds and Kelvins/Celsius or English system or any other system. In using basic new basic physical units, MM, LL, tt, and θθ or the old system relieves the discussion from using particular system measurements. The density, for example, units are Mass/Length3Mass/Length3 and in the new system the density will be expressed as M/L3M/L3 while in S.I. kg/m3kg/m3 and English system it slug/ft3slug/ft3. A common unit used in Fluid Mechanics is the Force, which expressed in SI as Newton [NN]. The Newton defined as a force which causes a certain acceleration of a specific mass. Thus, in the new system the force it will be defined as MLt−2MLt−2. There are many parameters that contains force which is the source reason why the old (or alternative) system use the force instead the mass. There many physical units which are dimensionless by their original definition. are the angle, strains, ratio of specific heats, kk, friction coefficient, ffand ratio of lengths. The angle represented by a ratio of two sides of a triangle and therefore has no units nor dimensions. Strain is a ratio of the change of length by the length thus has no units. Quantities used in engineering can be reduced to six basic dimensions which are presented in Table 9.1. The last two are not commonly used in fluid mechanics and temperature is only used sometimes. Many common quantities are presented in the following Table 9.3.


Table 9.3 Physical units for two common systems. Note the second (time) in large size units appear as "ss'' while in small units as "secsec."

Standard System Old System
Name Letter Units Name Letter Units
Area L2L2 [m2m2] Area L2L2 [m2m2]
Volume L3L3 [m3m3] Volume L3L3 [m3m3]
Angular Velocity 1t1t [m3m3] Angular Velocity L3L3 [m3m3]
Acceleration Lt2Lt2 [msec2][msec2] Acceleration Lt2Lt2 [msec2][msec2]
Angular Acceleration 1t21t2 [1sec2][1sec2] Angular Acceleration 1t21t2 [1sec2][1sec2]
Force

MLt2MLt2

[kgmsec2][kgmsec2]

Force

Ft2LFt2L

[Ns2m][Ns2m]
Density

ML3ML3

[kgL2][kgL2]

Density Ft2L4Ft2L4 [Ns2m4][Ns2m4]
Momentum

MLtMLt

[kgmsec][kgmsec] Momentum FtFt

[Nsec][Nsec]

Angular Momentum ML2tML2t [kgm2sec][kgm2sec] Angular Momentum LFtLFt [mNsec][mNsec]
Torque

ML2t2ML2t2

[kgm2s2][kgm2s2]

Torque

LFLF

[mN][mN]

Absolute Velocity

MM1t2MM1t2

[kgms][kgms]

Absolute Velocity

tFL2tFL2

[Nssec][Nssec]

Kinematic Viscocity

L2tL2t

[m2s][m2s]

Kinematic Viscocity

L2tL2t

[m2s][m2s]

Volume Flow Rate

L3tL3t

[m3sec][m3sec]

Volume Flow Rate

L3tL3t

[m3sec][m3sec]

Mass Flow Rate

MtMt

[kgsec][kgsec]

Mass Flow Rate

FtLFtL

[Nsm][Nsm]

Pressure

MLt2MLt2

[kgms2][kgms2]

Pressure

FL2FL2

[Nm2][Nm2]

Surface Tension

Mt2Mt2

[kgs2][kgs2]

Surface Tension

FLFL

[Nm][Nm]

Work or Energy

ML2t2ML2t2

[kgm2s2][kgm2s2]

Work or Energy

FLFL

[Nm][Nm]

Power

ML2t3ML2t3

[kgm2s3][kgm2s3]

Power

FLtFLt

[Nmsec][Nmsec]

Thermal Conductivity

MLt3θMLt3θ

[kgms3K][kgms3K]

Thermal Conductivity

FLtFLt

[NsK][NsK]

Specific Heat

L2t2θL2t2θ

[m2s2K][m2s2K]

Specific Heat

L2Tt2L2Tt2

[m2s2K][m2s2K]

Entropy

ML2t2θML2t2θ

[kgm2s2K][kgm2s2K]

Entropy

LFTLFT

[mNK][mNK]
Specific Entropy

L2t2θL2t2θ

[m2s2K][m2s2K]

Specific Entropy

L2t2TL2t2T

[m2s2K][m2s2K]

Molar Specific Entropy

ML2t2MθML2t2Mθ

[m2s2Kmol][m2s2Kmol]

Molar Specific Entropy

FL2TMFL2TM

[NmKmol][NmKmol]
Enthalpy

ML2t2ML2t2

[kgm2s2][kgm2s2]

Enthalpy

LFLF

[mN][mN]

Specific Enthalpy

L2t2L2t2

[m2s2][m2s2]

Specific Enthalpy

L2t2L2t2

[m2sec2][m2sec2]

Thermo Dynamic Force

MLt2MMLt2M

[kgms2mol][kgms2mol]

Thermo Dynamic Force

NMNM

[Nmol][Nmol]

Catalytic Activity

MtMt

[molsec][molsec]

Catalytic Activity

MtMt

[molsec][molsec]

Heat Transfer Rate

ML2t3ML2t3

[kgm2sec3][kgm2sec3]

Heat Transfer Rate

LFtLFt

[mNsec][mNsec]