5 Experimental Fluid Mechanics
339
and the temperature of the upper substrate was lower than that of the lower
substrate, and the temperature difference was constant at T . When T is relatively small, that is, the temperature difference between the upper and lower
plates is relatively small, and the liquid in the system is in a non-circulating
state. However, due to the thermal expansion and contraction effects of the
liquid, the liquid close to the lower plate causes a decrease in density due to
an increase in temperature, which is smaller than that of the liquid above.
Due to the gravity of the liquid itself, there is a downward flow trend for
the liquid with high density on the top of the container. As the temperature difference between the upper and lower plates continues to increase, the
difference in liquid density between the upper and lower layers also increases.
When the temperature difference reaches a critical value, the gravity of the
liquid overcomes the viscous diffusion of the liquid, and the liquid that is
initially in stability changes, causing the liquid to convectively flow within
the container.
5.2 Similarity Principle
Fluid dynamic tests (aerodynamic and hydrodynamic tests) are generally
divided into physical tests and model tests. Physical tests (such as aircraft
flight tests and missile live-fire launch, various prototype observation tests)
do not cause the distortion of model and environment simulation, and have
always been the means to finally identify physical flow dynamics and observation flow fields, but the test costs are expensive and the test conditions are
difficult to control. The model test is carried out under the condition of artificial control using the model which is similar to the real object geometry.
In order to make the model test results applicable to the actual situation, it
is necessary to make the flow around the model similar to the flow around
the real object. In this way, their dimensionless hydrodynamic characteristics
can be the same. This requires the same ratio of homogeneous forces acting
on the volume element at all similar corresponding points. In fluid dynamic
tests, the ratios of these dimensionless numbers are called similar parameters. There are many similar parameters, such as Mach number, Reynolds
number, Froude number, and so on. In a model test, it is difficult to make all
parameters exactly equal to the real object. However, for a specific model
test, the functions of similar parameters can be divided into primary and
secondary, and the similar parameters to be simulated should be determined
according to the purposes, requirements, and other specific conditions of the
test.
339
and the temperature of the upper substrate was lower than that of the lower
substrate, and the temperature difference was constant at T . When T is relatively small, that is, the temperature difference between the upper and lower
plates is relatively small, and the liquid in the system is in a non-circulating
state. However, due to the thermal expansion and contraction effects of the
liquid, the liquid close to the lower plate causes a decrease in density due to
an increase in temperature, which is smaller than that of the liquid above.
Due to the gravity of the liquid itself, there is a downward flow trend for
the liquid with high density on the top of the container. As the temperature difference between the upper and lower plates continues to increase, the
difference in liquid density between the upper and lower layers also increases.
When the temperature difference reaches a critical value, the gravity of the
liquid overcomes the viscous diffusion of the liquid, and the liquid that is
initially in stability changes, causing the liquid to convectively flow within
the container.
5.2 Similarity Principle
Fluid dynamic tests (aerodynamic and hydrodynamic tests) are generally
divided into physical tests and model tests. Physical tests (such as aircraft
flight tests and missile live-fire launch, various prototype observation tests)
do not cause the distortion of model and environment simulation, and have
always been the means to finally identify physical flow dynamics and observation flow fields, but the test costs are expensive and the test conditions are
difficult to control. The model test is carried out under the condition of artificial control using the model which is similar to the real object geometry.
In order to make the model test results applicable to the actual situation, it
is necessary to make the flow around the model similar to the flow around
the real object. In this way, their dimensionless hydrodynamic characteristics
can be the same. This requires the same ratio of homogeneous forces acting
on the volume element at all similar corresponding points. In fluid dynamic
tests, the ratios of these dimensionless numbers are called similar parameters. There are many similar parameters, such as Mach number, Reynolds
number, Froude number, and so on. In a model test, it is difficult to make all
parameters exactly equal to the real object. However, for a specific model
test, the functions of similar parameters can be divided into primary and
secondary, and the similar parameters to be simulated should be determined
according to the purposes, requirements, and other specific conditions of the
test.