1. Resonance
When a system is subjected to external excitation, its amplitude of forced vibration can become very large if the frequency of the excitation is close to one of the system’s natural frequencies. This is referred to as resonance.
Systems have many natural frequencies, but we usually focus on the frequencies in the lower range.
In physics, resonance refers to the phenomenon of two objects with the same vibration frequency causing the vibration of a third object when one of them vibrates.
The term “resonance” is also used in mechanics to describe the phenomenon where an object produces sound due to vibration at its resonant frequency.
For instance, when two tuning forks with the same frequency are placed near each other, one will produce sound when it vibrates and the other will also start to vibrate and produce sound.
2. Vortex vibration
Vortex vibration refers to the vibration caused by the alternation of vortex shedding after the flow around a solid body under the influence of mean wind.
The study of vortex vibration in bridges is a field within aerodynamics.
Vortex-induced vibration of bridges is a type of vibration that has both self-excited and forced vibration characteristics, with finite amplitudes.
It can maintain the vortex-induced frequency constant within a wide range of wind speeds, resulting in a “lock-on” phenomenon.
The calculation of the finite amplitude of bridge vortex-induced resonance is a crucial but challenging problem.
Currently, a comprehensive theory for the analysis of bridge vortex vibrations has not been fully developed both domestically and internationally.
In practice, a combination of semi-theoretical and semi-experimental methods is used to approximate the amplitude of vortex-induced resonance.
3. Flutter
Flutter refers to a self-excited vibration phenomenon caused by the interaction between aerodynamic forces and the elasticity and inertia of the structure. It is a result of the coupling between the flow and the structure.
Buffeting, on the other hand, refers to the forced response of a structure to periodic aerodynamic forces caused by unsteady flow conditions, such as flow separation and shock boundary layer interference.
Therefore, under the traditional definition, classical flutter is a type of self-excited vibration, while buffeting is a type of forced vibration.
There is also a phenomenon known as stall flutter, which occurs at high angles of attack.
Some experts believe that this type of structural vibration, which is characterized by strong separation conditions, coexists with flutter and buffeting.
4. Buffeting
Buffeting in aircraft refers to the vibration of aircraft components due to the excitation of separated air flow or wake, causing them to oscillate at their natural frequency.
One common example of buffeting is tail wing buffeting, which occurs when the tail is in the wake of the wing, fuselage joint, or other components. The disturbance in the wake causes the tail to vibrate strongly.
High angles of attack can make an aircraft particularly prone to tail flutter, which has been the cause of serious accidents in the past.
The wing can also experience buffeting due to the separation of its own air flow. In the transonic range, shock wave-induced boundary layer separation is another important cause of buffeting.
Buffeting imposes limits on the available lift coefficient and Mach number of the aircraft. To prevent buffeting, the aerodynamic shape is typically corrected and the relative position between the tail, wing, and fuselage is properly arranged.
Buffeting is a random vibration, but it is regular in the frequency domain, and the main peak of its power spectrum usually corresponds to the first natural frequency.
While buffeting does not immediately damage the structure of the aircraft, it increases the structural stress, reducing the fatigue life of the aircraft. It also has a negative impact on the aerodynamic performance, weapon system, mechanical and electronic instruments and equipment, as well as passenger comfort.
In severe cases, buffeting can cause the pilot to lose control, endangering the safety of the flight and the pilot.
Therefore, buffeting is considered an important factor in aircraft design.
5. Surge
Surge is an abnormal vibration that occurs in a turbine compressor, also known as a vane compressor, when the flow decreases to a certain level.
Centrifugal compressors, which are a type of turbine compressor, are particularly vulnerable to surge.
The occurrence of surge is related to the characteristics of fluid machinery and pipelines. The larger the capacity of the pipeline system, the stronger the surge and the lower its frequency.
Surge disrupts the regular flow of the medium inside the machine, creates mechanical noise, causes strong vibration of its components, and accelerates the wear and tear of bearings and seals.
If surge causes resonance in the pipeline, machinery, and its foundation, it can result in serious consequences.
6. Galloping
Galloping is a type of vibration that occurs in structures with complex and irregular non-streamline sections, such as square, rectangular, and other similar shapes.
The cause of galloping is that the lift curve has a negative slope, which creates a negative damping effect on the air lift, causing the structure to continuously absorb energy from the outside and form a divergent vibration similar to flutter.
Based on the mechanism of generation, galloping can be divided into two types: wake galloping and cross flow galloping.
Wake galloping is an unstable vibration caused by the downstream structure being excited by the flow passing by the fluctuation of the front structure. Structures such as the cables of cable-stayed bridges and suspenders of suspension bridges are most susceptible to wake galloping.
Cross flow galloping is a divergent bending self-excited vibration caused by the negative slope of the lift curve. This negative slope causes the displacement of the structure to align with the direction of the air force during vibration, causing the structure to continuously absorb energy from the outside and result in unstable vibration.
Cross flow galloping typically occurs in flexible lightweight structures with angular non-streamline sections, such as cables and suspenders in suspension bridge systems.
There is also the possibility of galloping divergence in other structures, such as girder steel bridges with a small width-to-height ratio, high and flexible long-span cable-stayed bridges, suspension bridge towers, and main beams of continuous steel frame bridges during the maximum cantilever construction stage.
7. Vortex street
Vortex street is a common phenomenon in fluid mechanics that is often observed in nature.
When a steady incoming flow passes around objects under certain conditions, vortices with opposite rotational directions and regular arrangements will periodically shed from both sides of the object, forming a Carmen vortex street after nonlinear action.
For example, if water flows past a pier or wind blows past a tower, chimney, or electric wire, a Carmen vortex street will form. The phenomenon is named after Carmen, who first proposed its existence.
Prominent Chinese mechanical engineers Qian Xuesen, Guo Yonghuai, and Qian Weichang have all worked in the Carmen laboratory.
If the alternating shedding frequency of the vortex street coincides with the acoustic standing wave frequency of the object, resonance will occur.
Many industrial preheaters and boilers are composed of circular tubes, and the fluid flowing around the circular tube can cause the alternating shedding of Carmen vortex street to vibrate the gas column in the preheater box.
If the alternating shedding frequency of the vortex street coincides with the acoustic standing wave frequency of the object, it can cause acoustic resonance and cause the tube box to vibrate violently. In severe cases, the preheater tube box’s vibration drum may become unsteady or even break.
To prevent damage to the equipment, the natural frequencies of the pipe box and gas can be adjusted to stagger them from the shedding frequency of the Carmen vortex street, avoiding resonance.
