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A subsonic wind tunnel refers to a wind tunnel with an airflow Mach number ranging from about 0.4 to 0.8 in the test section, and the lower limit of the Mach number can also be as low as 0. The structure of the subsonic wind tunnel is similar to that of the recirculation low-speed wind tunnel, and the power of the driving fan is larger than that of the low-speed wind tunnel of the same size. High cooler or ventilator. There are not many such wind tunnels in the world, and most of them were built before the birth of transonic wind tunnels and trisonic wind tunnels (wind tunnels operating in sub-, trans- and supersonic speeds).

Cooling Issues in Subsonic Wind Tunnels
The freezing problem of subsonic wind tunnel is a very important problem in wind tunnel design. When the wind tunnel is working normally, all the energy supplied to the wind tunnel by the power unit is converted into heat energy through friction, separation, etc. In addition to a part of this heat energy being transferred to the surrounding environment, the temperature of the wind tunnel airflow and the tunnel equipment will be increased until an equilibrium temperature is reached. If you do not manage to dissipate this heat, the temperature will increase over time and reach a very high equilibrium temperature. This has a very adverse effect on both the wind tunnel and the test. These impacts mainly include the following aspects:
(1) If other test conditions remain unchanged, the increase in temperature will increase the viscosity coefficient of the air, thereby reducing the Reynolds number of the test, and even causing the flow state near the model to change.
(2) Excessive test temperature will soften the material used to temporarily fill the shape on the surface of the model, such as wax, thus changing the shape of the model.
(3) If the internal strain balance is used, the high temperature will cause the deformation of the balance itself and affect the working conditions of the internal resistance wire of the balance. For mechanical balances, excessive temperature changes can also cause the zero reading of the balance to drift, thus affecting the measurement accuracy.
(4) The wind tunnel structure will expand due to the increase in temperature, thus causing thermal stress or deformation inside the structure. If this factor is not considered in the design, the deformation can even cause structural failure.
(5) If the motor driving the fan is installed inside the wind tunnel, and the motor itself does not have special cooling equipment, the temperature rise will deteriorate the working performance of the motor, or even cause damage.
Among the above effects, Articles (1) and (4) are particularly noteworthy. From this point of view, the cooling problem of the subsonic wind tunnel is very necessary and important. Generally, the temperature of the test section is expected to be constant and kept between 15-20 °C, and the highest should not exceed 45 °C.
There are roughly three methods used for wind tunnel cooling:
(1) Internal cooling method of wind tunnel;
(2) Surface cooling method of wind tunnel pipe;
(3) Air exchange cooling method.
Which cooling method is used in a wind tunnel should be considered from many aspects. For example, the structural material and heat transfer of the wind tunnel pipeline, the air flow to be cooled and the temperature difference between inside and outside the wind tunnel, whether the pressure or density of the wind tunnel changes, the tests undertaken by the wind tunnel, and the local construction conditions, etc. No matter which cooling method is used, the cooling system should be mainly installed at the highest temperature of the wind tunnel pipe, and the pressure loss caused by the cooler should be minimized.

Structural Issues in Subsonic Wind Tunnels
In subsonic wind tunnels, since large diffusion ratios are generally used, and large power equipment and coolers need to be installed, the diameter of the wind tunnel pipes is relatively large and the pipes are relatively long. The load and heating of the wind tunnel body are very serious. Therefore, many issues must be considered when designing the wind tunnel structure. These issues are related to the following wind tunnel aerodynamic parameters and conditions.
(1) Whether the wind tunnel has changed the pressure and the scope of the change;
(2) Static pressure and dynamic pressure near the wind tunnel test section;
(3) The internal air temperature and working environment of the wind tunnel during operation and parking;
(4) Vibration caused by fan rotation and air flow, etc.

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